Personally I would down play it to her as much as possible. if you let her stay off, then it will be much worse getting her back to school-any school.
As the teacher spoke to you, I wouldn't expect a slip as well. A slip is basically to explain bruise/cuts and to suggest you check that all is okay later (esp. a bump on the head).
I wouldn't expect a slip for that sort of incident, but it was right of the teacher to speak to you-teacher speaking to you ime means it is more serious than a slip. Unless she was inconsolable, again I wouldn't have expected a call.
Was she upset about it happening, or was she upset because she thought she was in trouble?
At 5yo they often don't have friends. The question is whether she is choosing to be on her own-in which case she can be gently encouraged to join in with others; or the others are excluding her, which can be bullying. Ime strong friendships only start for the most part in year 1, and often not until year 2. Have you tried inviting friends back after school?
One of my dc excludes herself, she'll choose to sit on her own, and even refuse invitations to sit with others because she lacks confidence in that way.She is actually quite popular in a quiet way-she's the one they choose to go to for help and support.
I doubt they will be able to move class unless there's space in the other class (ie less than 30). But this happened at lunchtime, so being in class with them is slightly irrelevant.
It may be nothing like as sinister as you think it is. I remember at year 1 level playing houses, and one girl said to another, "we're going to bed now, so get undressed" and us watching in surprise as the other girl unbuttoned her dress. We meant "pretend to get undressed" she thought we were really doing it, we stopped her pretty quickly, but what I'm meaning is that it wasn't necessarily a malicious incident.