Middle of lower back hurts very bad why? It started hurting me bad today at school, so bad that it hurts to bend. Haven't don't exercise lately. 16yrs.

Trauma or infection? Not sure why your descriptor shows you as 19yo when you stated 16yo. You didn't mention any trauma or injury but that ranks up there especially if you're a victim of bullying. Back is more worrisome if any numbness, tingling, weakness or loss of bladder/bowel control. You also didn't mention painful urination which would make your back pain more worrisome for kidney infection. Go see family doc.