No AUP will stop a determined student from locating and displaying material beyond that which is appropriate in schools. The best advice one can carry into meetings to develop an AUP is always to remember the Student Code of Conduct. Virtually everything a student can do on the Internet can be done in a similar manner in non-computer fashion. If your Student Code of Conduct is sufficiently robust to cover these incidents, it is sufficiently robust to handle student access to the Internet. Pornography is the same, whether it comes into the campus in a magazine or on a computer monitor.
Sexual harassment is the same in the halls and on e-mail.
Vandalism is vandalism, from spray painting bathroom walls to crashing the campus computer network. Making a few minor changes in wording can enlarge the Code of Conduct to include network events in its scope. Keep the Code in mind while constructing the AUP and you will prepare a seamless educational network environment for your students.