I've have been thinking about this movie off and on for years, so I hope someone can help me. All I can really remember is there's some guy that I believe is in high school, but he turns into some kind of monster, but he's a good monster and the principal or dean of the school turns out to be the head bad monster. The high school kid looks like "Nick" from Family Ties, but apparently it's not him. Anyone have any ideas.

Teen Wolf?

I'm going to agree with Ghost that it's "Teen Wolf". Michael J. Fox played the Wolf and main character so that's probably why you are remembering Family Ties.