Leaving under what conditions? After having incited other people to violence or disorderly conduct? Leaving perhaps as a trespasser? Leaving perhaps with school property?
Trespassing and inciting people to violence are against the law. I believe the officers are well within their rights to stop someone and identify them especially if they may end up arresting him for breaking the law.
If I ask someone legitimately for ID and suspect them of trespassing, and then they tell other students to join him, then shame on him.
If I ask someone who has broken the law to stop and talk with me and he keeps walking, he might get tazered. Well, with me it would more likely result in me manipulating his elbow. But, with campus security, they are probably stuck having to use tazers.
What about 'refused' to show his ID? That is a step beyond not carrying ID. Bad decision on his part -- it is a simple request. Maybe it interfered with a plan he had....