Harvard has three kinds of law people. First, it has its own lawyers. They're fine, but they're mostly not the court kind of lawyers. Second, it has law students. Those aren't lawyers, so you don't want them either. Third, it has law professors, who are generally well known or renowned for past deeds or theoretical legal philosophy, but mostly they're either out of practice, never practiced, or they've got insane (for example, Alan Dershowitz is a professor there). You don't want any of those folks nearby when courtroom stuff needs to happen.
Hogwarts has a similar problem, now that I think about it. Hugely powerful wizards defending a school from a tiny number of adversaries, they've got all the time in the world to prepare, and yet they're terrible at it. They build a huge multilayered vault that's bypassed by three unprepared kids. They should've brought in professionals.