What are these knee problems I'm having?

I'm in a mildly intense summer Physical Education cl. I wake up at six o'clock in the morning and get to the cl's headquarters at seven. I do a tough workout including some sort of "extreme" game (extreme volleyball, extreme baseball, extreme basketball, et cetera), weight lifting (bench pressing, curls using dumb bells, et cetera), followed by a cardio session (in which we will run a mile, walk two or three miles, do "community service", or some other activity to get our hearts pumping). Monday, our cl ran up and down row after row of metal bleachers, followed by brisk walking and then repeating the entire process over again two more times. I go to sleep fairly early, getting enough healthy rest, but, today (Tuesday), when we started running, I had sharp pains in my knee area, immediately below my kneecap. They have not gone away since then (around nine o'clock in the morning). I don't have any known family cases of knee problems, but my knee hurts nonetheless. I'm thin, so there isn't much weight I'm putting on my legs or feet, but I am pretty out of shape. My parents won't buy me a knee brace until the pain persists, and my dad tells me masking the pain with painkillers won't work, because inside, it'll only let things get worse. My question is: "What's wrong?" Also, I have three more days of the cl (Wednesday, Thursday, and Friday), and one absence equals three zeros in my grade book. What should I do?