Continuity mistake: Just before Jessica goes and asks the woman with the pram if she'd like some lemonade, we see Nicki put the plastic cups down beside the 'Cash' box. In the next shot of Jessica talking to the woman, the cups are on the other side of the table. They're back in their original place in the next shot.
Continuity mistake: When the security guard enters the room where Brian had hacked the computer, he's out of sight, having hidden in the vent above the room. But he was sitting in the chair just moments before as the guard was opening the door. He couldn't possibly have made it up there in time.
Audio problem: When Brian escapes his room and the headmaster with security is checking the rooms, if you look very carefully while the camera is going to Brian's room, you can see the headmaster hitting the door with his hand out, but the sound sounds like it would if he were hitting it with his fist.