A Teen in a grim reaper's robe walked in. Teachers tried to make him take off the robe, but he simply shooed them away. They of course would not, so he did some meddling. He carried around a strange tech box that he would push buttons on and something would happen(lights turn off, sprinklers turn on, class bell rings, etc.) he used "lights out" most often.
He would escape with whatever method he chose. He headed straight for the water fountain. Once he reached it, he poured it into several vials, a container, a canteen, some filling his plastic-lined backpack(no books or anything inside), and in a bottle. (Of course he knew about the fountain, why else would he go through all this trouble?)
When finished, he turned around to see a teacher standing over his shoulder. He immediately screwed off the cap of the bottle and poured on the teacher's head.
T:UGH!(with progressively youthening voice, body, and clothes) Why have you done this? I don't know what the rules are where you come from, but it's rude here. I should report you to the principal for this! why did you--
The teacher looked down in the puddle to see a two year old version of themself, fully clothed in young children's clothes.
T:AAAAAAAAAAHHHHHH!
The ex-teacher ran off.
?:*smirks and chuckles*
He refilled the bottle and walked into the school, containers in his water-filled bag, wondering who to get next. (he doesn't talk much.)