I only saw this twice, in the PS2. Coincidentally, she was fighting agaisnt Mandy outside the girls dorm both times.
In the first time, Mandy hit her and ran to behind the dorm. Beatrice started to chase her and pulled out the bottle launcher. She started to fire, but missed every shot.

The second time, she got the launcher and fired almost immediately after they spawned, knocking Mandy out.

Its interesting that the prefects didn't started to chase her for violence against a girl.