because it was too devastatingly large a portion of their students to expel them all, even though that was their stated policy

And here we can observe the mechanism behind collective action: Whether what you're doing is legal is becomes less important if there are enough of you to make punishing them all infeasible.

Of course, reality is more complex and less homogeneous than a class of aspiring lawyers, so assembling a sufficiently large and disciplined group is more difficult.