This is/was "hotly debated". IMO public schools are like any other public thing and accessible to vamps generally, and Angel was just being sassy when Jenny asked him how he got in.

But along those lines, my initial response was "Depends on who's writing" with the Angel/Jenny school scene in mind. "Enter all who seek knowledge" or whatever.

Overall they were not very consistent when it came to the invitation thing.

That sounds like something a vampire hunter would say