Caio gets a tray of food - mostly brussels sprouts but he saw other people taking the rolls and the pie so he reckons those are safe and has some too, plus a carton of milk. Casts about for someone to eat with who he has... met. If he has to go eat with the school nurse no one will ever think he is cool. There's Camillo. Camillo is an enclaver but maybe Caio made an acceptable impression?
How to approach. Not "is this seat taken", that puts people on the spot. "Can I sit here" is very direct. He's overthinking this. "Hey, um," he says, approaching an empty chair and trying to catch Camillo's eye, "um, uh?" Point at chair. "May I?"