No smoke without fire

In our boarding house at school, we have a number of red water fire extinguishers. These are about 10 kg each, and have a little tag that inserts into the handle to stop you from accidentally setting them off. It seems that quite often the tags go missing, but as long as the handle is depressed the fire extinguisher is still servicable and doesn't have to be reprocessed. So this evening I was somewhat bemused when two kids I caught messing about with a fire extinguisher seemed to think that the fact the tag was missing somehow absolved them from all blame, and that I was being unreasonable in going ballistic at them! I don't think they understood quite how serious an offence they were committing. IIRC, it's a criminal offence in this country to tamper with firefighting equipment.