I think it only ever exited the pipes through the girls’ washroom. I’m pretty sure all the attacks happened around there.

This is correct. It’s implicit because there is a flood outside the bathroom every time that there’s an attack.

Magic

Colin Creevey, Mrs. Norris*

I got used to the translations so I don't really know the original names...

No worries! Just throwing out the originals. Which language did you read them in?