No one is that busy/important that they can't spare a few seconds to let someone know they're not needed that day.
You've never been in a school, right? 8:55, as the students are already coming in, putting bags away - Miss look what I got at football last night, Miss I found a rabbit in the park this morning, Miss I lost a tooth, Miss mum wants to talk to you about my homework, Miss I can't find my drink bottle - two parents are standing arounding wanting to talk to you about something, the year coordinator puts their head in the door telling you that they've decided to hold practice this morning in the hall instead of reading groups, another parent wants to talk, bell goes, you get the kids in their seats, take attendance, get Joey down from the top of the bookshelves where he is looking for the pen he threw up there yesterday, try and get all the notes and money organised, send a student to the office, make sure everyone knows about the change in schedule for the morning (including trying to talk the two children with special needs through the sudden changes to their schedule that they won't cope with), making sure the TA knows not to let the two children run off due to the change in schedule, get everyone to get their water bottles, get everyone lined up at the classroom door, walk students through the school, trying to keep quiet to avoid disturbing other classes, get to the hall, find that no one has the key, send someone to the office, get the hall open, get students inside and lined up in something resembling rows, talk to the other teachers to find out WTF is happening and who is supposed to be running this rehearsal, try and stop A from hitting B, try and stop C from moving from his spot, try and remember who is in which role for the performance, get students in to their groups, and then you can actually begin the rehearsal that was never in your schedule and try and work out how you're going to fit in the missed literacy session.
But sure, in the middle of all that, we'll just drop everything and go and find a phone number for Sally's mum, hope she answers her phone, and tell her that things have changed and she needs to go to the hall or to another classroom for the morning. Or Sally's mum could use her initiative, ask and find out the classroom is in the hall, go there and actually be useful, instead of sitting around sulking that no one remembered her.