S&W .40 Shield. Underwood .40 ammo is perfect for such a situation. No need for lower or higher power cartridge. I would also carry spare magazines.

I worked for four years at a secure facility for extreme at risk teen girls. Perimeter doors were kept locked. The environment was very tense; always on edge. Qualified professional staff only, yet turnover was very high. All staff received 8 hours of Emergency Safety Response training every four months. Staff were unarmed, and the police were called frequently. All of the students were court ordered, no private referrals.