The three districts accounted for almost 60 percent of the near-2000 total number of children who were asked to take the precautionary step across Lancashire in the week to 28th March.
Figures obtained by the Post show that 1.1 percent of the school population in the Lancashire County Council area - which excludes Blackpool and Blackburn - was advised that they should self-isolate.
More pupils were isolating in Chorley than any other Lancashire borough - 494 children, representing just over three percent of the school roll in the area. The majority - 410 - were secondary school students.
That split was reversed in Preston the area with the second-highest number of isolations last week - 364 out of the total 425 pupils in the city who were staying away from class were primary school age. In South Ribble, 229 youngsters were in self-isolation, while elsewhere in Lancashire, five districts reported fewer than 100 isolations.
However, the number of confirmed infections was low - 84 across the entire county council area, just 0.05 percent of all pupils. Preston had the most - 18 - while South Ribble had the third-highest with 15.
Only two Lancashire schools were completely closed because of outbreaks last week, but 53 were affected by isolations - 13 of them in Preston, more than double any other part of the county.
Ninety-one staff had been told to self-isolate over the seven-day period, with Preston again having the highest tally at 26. It also had the joint-highest number of staff infections, along with Chorley - with each recording five confirmed cases.
Preston City Council leader Matthew Brown said the figures reaffirmed his belief that the return to school on 8th March should have been a more gradual one.
“I’ve said all along that if we get these decisions wrong, we’re putting lives at risk. I genuinely hope that it’s me that’s wrong and that this won’t be an issue.
“But my anxiety is that we have not got through vaccinating enough people yet for it not to be a public health risk.
“We’ve got the two-week holidays now, so let’s hope that will act as a circuit break.
“However, I think the government needs to give local authorities the flexibility so that if they do think there is a risk, they can stage the return [after Easter],” Cllr Brown said.
In the week to 7th March, the last week during which schools were open only to vulnerable pupils and the children of key workers, 223 pupils in the county council area were told to self-isolate - just over a tenth of the figure for the most recent week.
Government guidance is currently for all secondary pupils and staff to carry out twice-weekly lateral flow tests at home, which provide results within the hour.
If a test is positive, the individual, their household and close contacts must begin self-isolating immediately. As of 30th March, they can book a lab-analysed PCR test for confirmation of the result.
The guidance states that entire classes do not have to isolate in the event of a positive test among their number, but rather those who have had face-to-face contact with the infected individual or been within a metre of them for a minute or more or within two metres of them for at least 15 minutes.
Primary school pupils are not being asked to undertake the twice-weekly testing - although staff are eligible for it - and children should still get a test if they develop symptoms.
A spokesperson for the Department for Education said in response to the latest Lancashire data: “Attendance in schools remains higher than at any point during the autumn term, as students and staff continue to follow the protective measures set out in our guidance to reduce transmission of the virus.
“With the testing programme now in full swing and millions of tests being conducted each week, we are only seeing a small increase in the numbers of students testing positive and self-isolating.
“We are grateful to everyone who continues testing twice-weekly from home and self-isolating where that is necessary, playing their part in keeping everyone as safe as possible.”
Meanwhile, Lancashire’s largest teaching union is calling on education bosses in the county to specify the “trigger points” that would see schools advised to return to remote learning.
Local authorities have received guidance from the government asking them to help schools to take “a proportionate approach to responding to positive cases in their settings”.
The National Education Union (NEU) is now seeking clarity from Lancashire County Council about whether such measures would include homeschooling in areas where Covid case rates had reached a certain level - and what that level would be.
“We are all set up for that now - nobody wants it, but if there is a spike somewhere, then let’s act so we can stop community case rates from going up and learning can continue,” said Ian Watkinson, the Lancashire representative on the NEU’s national executive and chair of the union’s health and safety group.
“There is a fear about putting a number on it and I understand that - but it would be helpful to know.”
He said that the government should have moved to use space in public buildings to host classes and allow for more effective social distancing until more of the population is vaccinated.
“Most schools don’t have the money to fit specialist filters or high-tech ventilation systems and so where they are at full capacity and not allowed to operate rotas, the cases are just going to keep on rising - that is exactly in line with what the science predicted.
“Everyone wants things to get back to the way we were - teachers included. So we have just been getting on with it - teachers have been glad to see the kids and vice versa.
“Headteachers are doing their absolute best, but they cannot stop the virus coming into schools. When cases occur, we’re back to that disruption to learning - and to parents - because kids are isolating again,” Mr. Watkinson added.
In response to the union’s call to set intervention levels for action in schools, a spokesperson for Lancashire County Council said: "Schools have been committed and supportive in preventing the spread of coronavirus and all have appropriate risk-assessed measures in place.
“These, along with the support through the national testing for education settings and households, will help to identify asymptomatic cases early, enable close contacts to isolate, and minimise transmission of Covid-19.
"We are working closely with partners, schools and all other education settings to respond to any outbreaks in accordance with national guidance and have the systems in place to escalate the level of support within schools if necessary in order to manage them, while ensuring children continue to receive their education whether in school or at home."
The rolling weekly Covid case rate in the community across Lancashire has been on a downward trend throughout the last month, standing at 60.8 cases per 100,000 people in the week to 28th March.
Preston has seen a generally rising trend since 19th March, pushing the city back over the 100 case rate level - although it is falling again in the most recent data at 101.3 per 100,000 residents.
Chorley had a case rate of 71.9 over the same period, having fluctuated between rises and falls over the past month, while South Ribble, after an increase at the start of March, has since dropped to 62.3.
LANCASHIRE PUPILS ADVISED TO SELF-ISOLATE, 22ND-28TH MARCH
No. of pupils self-isolating in each district, with the proportion of the local school roll that figure represents in brackets, followed by the number of schools affected in that area.
Burnley - 191 (1.30%) - 4 schools (3 secondary, 1 primary)
Chorley - 494 (3.01%) - 5 schools (2 secondary, 2 primary, 1 pupil referral unit)
Fylde - 72 (0.86%) - 2 schools (1 secondary, 1 primary)
Hyndburn - 54 (0.41%) - 5 schools (2 secondary, 2 primary, 1 special)
Lancaster - 49 (0.25%) - 3 schools (1 primary, 2 secondary)
Pendle - 163 (1.10%) - 4 schools (all primary)
Preston - 425 (2.10%) - 13 schools (10 primary, 3 secondary)
Ribble Valley - 28 (0.26%) - 2 schools (1 primary, 1 secondary)
Rossendale - 105 (0.86%) - 6 schools (3 primary, 2 secondary, 1 special)
South Ribble - 229 (1.36%) - 3 schools (2 primary, 1 secondary)
West Lancashire - 117 (0.75%) - 3 schools (1 primary, 2 secondary)
Wyre - 29 (0.21%) - 3 schools (1 primary, 2 secondary)
LANCASHIRE COUNTY COUNCIL AREA TOTAL - 1,956 (1.11%) - 53 schools (29 primary, 21 secondary, 2 special, 1 pupil referral unit)
Source: internal local authority data