Hi,
... but Harry could see one person who didn't look happy at all. Percy was visible over the heads of some excited first-years, and he seemed to be trying to get near enough to start telling them off.
If a shorter person stands in front of you, you are able to see something that is behind that person by looking over the person's head. Or maybe the person is taller than you, and so you stand on a chair. Again, you can look over the person's head.
'To tell someone off' means to reprimand them, to express your annoyance, to complain about their behaviour.
Best wishes, Clive