I'd be surprised if it was someone at their school for 2 reasons:
- Abby and Libby didn't seem to recognise them
- I'm sure Police have asked members of staff at the school if they have any idea who it is in the video, I know it's not the greatest video but it's good enough surely for "That looks a bit like X"
I do agree that it would be a big conincidence that he is there, on a weekday when school would otherwise be in, kitted and attacking victims of that age.
That being said, I can think of many ways someone would be aware of it without working in the education system, anyone with children themselves, someone who has a colleague who isn't going to be in that day because they're taking care of their children, or has a family member or friend with children who mentioned it in a conversation could all make someone aware of it.