What if a student noticed his friend had been Taken and decided to hold whatever took his place hostage with some kind of trap of salt and iron, and traded the changeling's life for their friend back and and Oath that no harm coming from the Fae, directly or indirectly, would be made to those the student care about, including the student themself. What would happens?
It would violate the Treaty, and the student would be punished pretty severely by the school. Changelings are guaranteed safety by the faculty during their time on campus (so long as they aren’t actively harming anything else), in exchange for the safety of RAs at the end of each academic year when they wade into the Elsewhere to try and collect any missing students. Violating that treaty would put dozens of lives at risk on both sides, and more than likely start a cycle of revenge that could get very ugly very quickly if the University doesn’t nip it in the bud.