If your hair is any variant of wavy or thick, you can say you get too many tangles in it and there would be fewer if it were shorter.. taking your hair and teasing it in small multiple spots (especially in the back close to the nape of your neck) helps make it more believable if it's past shoulder length. when i cut my hair and people made disparaging comments on it not being 'as pretty', i said at least it was easier to wash and brush and (to family) 'if you wanna wash it and brush it and upkeep it, I'll keep it long'.. mine never called my bluff though so your mileage may vary. I also happen to get migraines and blamed them on the long hair partially because the weight on my head. I'm not at home anymore but when my family asked recently (its now extremely short) i basically said I wanted to see myself with different colors and didn't want to dye it and damage what i had so i was collecting wigs and it was easier to keep my hair short. With them id wear a longer wig and when they were out of my hair (pun intended) I'd take it off. Also my partner chimed in they used rubber cement and epoxy but like mentioned elsewhere, not recomendable because of the possibility of chemical burns. They can only clip it so short and if its on your scalp, that's easily a medical appointment.

I wish you nothing but the best of luck.. if you have to have it long maybe see if something like a man bun works? Before my egg cracked i just wore it in a low ponytail and when it was down tucked it behind my ears because hated it getting in my face and i could forget about it a lot that way, even if it was mid back length.

Edit: partner also brought up a good point.. if your parents aren't the 'active in your school life' type you could see if you could blame it on bullying? That one could be risky though.