The Vigils kept things under control. Without The Vigils, Trinity might have been torn apart like other schools had been, by demonstrations, protests, all that crap. (4.23)
The Vigils is the secret organization at Trinity high school. Everybody actually knows about it, but they pretend not to. Other than Carter, Archie, and Obie, we don't meet the members. As the quote up top demonstrates, they work hand in hand with Leon and other administrators to keep other students in a state of fear and submission. At the same time, they challenge authority by pranking teachers. The administration lets The Vigils get away with their pranks because they like using The Vigils to enforce their will, and tamp down possible student rebellions.