Southlands High School, Chorley is to shut from Monday June 21 and reopen on Monday June 28.
Head teacher Kerry Millar told parents in a letter dated yesterday (Friday June 18).
She wrote: "As I am sure you are aware, during the past 10 days we have been dealing with an ever-increasing number of positive Covid cases in students at Southlands High School.
"This has affected students in all year groups 7 – 10. At the end of today the number had reached 32 students.
"Firstly, I pass on our best wishes for speedy recoveries of our students and their families suffering the effects of Covid. Unfortunately, this results in a significant number of contacts per child who need to be contact traced.
"This has led to a vast number of students from various groups needing to go home and isolate across various dates."
Miss Millar said she and her senior team had monitored and managed the situation tirelessly and consulted Lancashire education chiefs and the school governors.
She continued: "It is with huge regret that I have to take the decision to close Southlands High School next week from Monday 21st June to Friday 25th June.
"This is to ensure we take action to reduce further spread of the virus and so that we can provide a full programme of live lessons to all students, for all lessons, including form time each morning.
"School will reopen on Monday 28th June."
She stressed that next week would be a normal working week, with staff and students working from home.