How do you sterilize a rooster to make a capon? I've asked several people and the best answer I've gotten was "the Ag teacher will do it".

If your rooster is older than dressing age (dual puropase breeds - 12 weeks) don't do it. It's not worth the trouble. If you want large tender birds, feed some Cornish X out to 12 or 14 weeks. With birds like the Cornish X available, it just isn't worth it to do surgery on a chicken. I'd be very surprised if the ag teacher can do it, either, based on some I've known. Very few ag school people have specialties in poultry, and caponizing usually insn't one of them.