How do you say no to a girl who wants you to model for pictures for school?

It sounds like there is more to the question than is stated. This would be a sensitive issue for, say, a college student who is being asked to do nude modeling for an art class on campus. There may be other settings that would cause you or others to hesitate, but they are not coming to mind. Whatever the situation, simply say that you choose not to model because you are concerned about privacy.

I am not even sure if a school would allow a student to do nude modeling on campus; it would be strange sitting nude for an hour in front of students and then to socializeor attendother classes with the same students.

If you are hesitating because you would like to develop a relationship with the girl, then just hold to your convictions. If she doesn't want anything to do with you for not agreeing to model, she is no great catch.