The data from Plan International UK shows that 35% of girls have received unwanted sexual attention or contact such as being groped, stared at, catcalled and wolf-whistled while wearing their school uniform in public.
One in eight girls reported being 12 years old or younger when she first experienced unwanted sexual attention.
The poll of 1,004 girls aged 14-21 also found that one in seven girls had been followed while in uniform, while 8% said they had been filmed or photographed by a stranger without their permission, or someone had taken a photograph up their school skirt.
Some 37% of the girls questioned said they had been been sexually harassed while travelling to or from school.
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Sixteen-year-old Jess, from Glasgow, said, "When I was 15, I was in school uniform and sat on a train and this guy kept trying to put his hand on my leg.
"I was like, 'What am I supposed to do?' I ended up getting off the train at the next stop and just being completely lost."
She added: "It was such a horrible experience. I was going to see my biology tutor and I arrived at the library in tears, I was really upset about it.
"I think the worst part was feeling guilty because I was wearing a skirt, which is stupid because it shouldn't matter what I was wearing, but in the moment it did."
The findings come as Plan International UK launches a new report into the impact of street harassment on girls and women in the UK.
The charity is calling on the Government to recognise harassment in public as a form of gender-based violence in its strategy to end violence against women and girls.
Tanya Barron, chief executive at Plan International UK, said, "It is shocking and deeply concerning that girls, many of whom are clearly of school age because they are in uniform, are being targeted and sexually harassed by perpetrators in the street.
"It's simply not acceptable that girls as young as 12 are being wolf-whistled at in public, touched against their will, stared at or even followed. This disgraceful behaviour needs to be called out and stopped."