Are you talking about for taking risque' pictures?
Technically, yes, 18 is legal age. You need to pay particular attention to
2257 if you are shooting more than artistic nudes... or even for that, too by some interpretations of the law.
BUT
The other thing to watch out for is there is usually a "student code of conduct" that they have signed... and you don't want to get on the bad side of a school district.
If you aren't talking about risque' pics... then the only reason I can think of to ask this question is about a model release... at 18 they can sign any legal document, including a model release.