Laws and such depend on where you live, of course. Here in Sweden if your school publicly posts photos of you and you wish to have them taken down, they’re obliged to do so. That was the case during PUL, which the GDPR superceded a while ago. I’d assume something similar should apply for you even if you’re not in Europe. The school doesn’t own your image, and has no right to use it if you don’t want them to.

It can be withdrawn in other cases too. If you agree to a medical procedure but change your mind, you can withdraw consent. If you enter intimate relations with someone but wish for that to stop, you can withdraw your consent. If consent to someone borrowing your car, but find that they’re not doing so responsibly, you can withdraw consent.

It’s all fairly normal. Your easiest avenue to have the photos removed would be to just ask the school. “Hi, I saw you posted photos of me on Instagram and it makes me a bit uncomfortable. Would it be at all possible to have them removed, or crop me out, please?”

I’ve always hated having my photos taken, so when schools organised that kind of thing I was always adamantly against being in them. Apart from some “oh come on!” and “it’ll be a nice memory!” they’ve always respected it.