I don't think the 1st Amendment requires you to open your school up to speakers that students don't like, nor does it require students to sit and listen or to not actively heckle. In fact, as far as I know, the only thing the first amendmendment does is prevent the government itself from silencing your speech. Are these kids or the school the government itself? Then the first amendment doesn't apply.
You are correct, the conduct of the hecklers, while contrary to the principle and spirit of free expression, is not prohibited by the First Amendment. It is contrary to Stanford's own written rules protecting open discourse; by refusing to enforce its rules, Stanford is violating their written committments.