I have lower back problems and need help?

im in 10th grade and since middle of ninth grade starting around 3rd hour sitting at my desk my lower back hurts real bad like right above my like where the 2 dimple looking things are and if i sit there it keeps hurtin and i cant move to wear it doesnt i have to stand but they dont let me stand up in cl anyone got an idea wat wrong with my back?? and what i can do??..... but when i stand up acouple seconds later it feels good agin