Simple. You take pictures of the principal having sex with the secretary and then show him the photos, and tell him that you expect A's for yourself and all your friends.How does one compromise a principle?
Oh. Wait. You said principle, not principal.
Okay, so you have some principle, and then you negotiate some or all of it away for some price that you feel is worth the compromise. Usually that price is a feeling that people won't think you're poopy, and you'll keep your seat next election. Another word for this process is "Republican".