He knows that no adult can supervise in the toilet block during school hours. (He places a yellow triangle in front of the door while he cleans) so the pupils can do what they like in there. And as you say Steve, grasses would get beaten up.
He has a special solvent to remove graffiti and the usual genitalia drawings, which shocked him terribly at first!
It's all very unhygienic.
I wonder if all schools would close if the virus should become rife?