@RhubarbTea
I can understand why you're concerned, it seems more real when you know someone who may have been affected. However, to think about it- their chances of having contact with one particular pupil in a large secondary school are pretty low. Last week, the pupil likely wasn't showing symptoms and so less likely to pass the virus on.
You say you didn't get that close to them, and it doesn't sound like they were ill, so I do think your chances of catching anything are low.
Obviously if it turns out they are considered "close contacts" of this pupil, things are different, and of course if you get a cough/temp etc then you should call 111, but at the moment your chances of having caught the virus are still relatively low.