YADNBU about the passport- as others have said, what if you want to go on holiday??
Before I talked to the school about anything else, though, I would sit down and have a long chat with my daughter. I would ask "If I were to contact your subject teachers and ask them whether all your homework had been handed in on time, what would they say?... no, no reasons or excuses why it wasn't done... just yes or no. And how about before you were staying with your dad?"
This is because it sounds like your daughter might have got more behind with her work than you might realise - to be in the library at every possible opportunity for two weeks sounds like a significant amount of homework missed - more than one or two weeks.
I realise that there were stressful circumstances and that your daughter's working conditions at her dad's were not ideal.
I do wonder why, while she was staying at her father's, she wasn't using the computers/library at lunchtimes or after school (when not in rehearsals) to keep up with her work?
AS levels are, in some cases, a big leap from GCSE. A* at GCSE can be from about 65% in some subjects (like maths) to about 95% for others (e.g. drama). Often pupils do find that there is a lot more work than they expect or are used to and can easily fall behind.
It sounds like this may have happened to your daughter and the school were concerned about its possible impact on her exam results.
So it might be better to have this information from her before you go in! Do you know what her mock exam results were like and what sort of grades she has been getting so far this year?