I have a similar problem
it's extremely difficult for me to find jeans that fit me properly in the waist (I have a big butt but a skinny waist). So all my jeans end up fitting in the thighs and rear, but being too loose in the waist. Belts don't even help, and end up making the waist part of my pants bunch up. If you're the person who wears jeans when not wearing your school uniform, I'd suggest jeggings or leggings being a part of your regular wardrobe. If you're worried about people noticing your rear maybe invest in some longer shirts/tunics? Again, no one has the right to harass you and if guys/men have, it should be reported.
Hope this helps!