If it is consensual, no problem. However, an anonymous tip saying "man with a gun" is not enough. There's actually a U.S. Supreme Court case that says so and with facts more favorable than those here. I can't think of the name of it off the top of my head. Students are in a different category and have diminished rights while at school, though I do not pretend to know much about the parameters of their rights.
My scenario is admittedly speculative. I'm just throwing it out there as a possible way out for everyone involved if they want to open that door.
Added: The case is
Florida v. J.L., 529 U.S. 266 (2000). There's a description of it at
http://www.fdap.org/downloads/articl...ymous_tips.pdf. Just read the first numbered section. BTW, the Supreme Court has just agreed to review an anonymous tip case.