Doug or Fozzer usually have a broad knowledge of these things so they may be able to help. But i am certain it isnt illegal to listen to ATC over the net.
I have no specific knowledge of the subject & until recently wasn't aware you could listen to any ATC on the Web. I suppose I should have realised but the thought never occurred to me. To the best of my knowledge, listening to civil ATC is perfectly legal both in the UK & the US. In fact many aircraft spotters use special "Aircraft band" radio receivers tuned to the local ATC so they know what's going on. I can't see what harm this does & I'm sure they would be stopped if there was anything illegal about it.*
Mind you, I wouldn't like to encourage just anyone to do it, especially on a mass basis - the whole of your school for example, unless it was a class project or something like that. There's some funny folk around these days but again, I can't see what possible harm it would do. Aircraft movements aren't exactly a big secret.
*PS. There are stands full of these things for sale at the average air show at airfields all round the country.