Anybody else get embarred when they have to do this?

we HAVE to wear tights with our uniforms at school its annoying i know. but anyway, they always seem to slide down even when i'm wearing the right size. so i have to excuse myself, go to the bathroom, hope nobody's looking, and hoist them up underwear and all. why could they be sliding down? might it be the material of the underwear? what can I do to prevent this?