We went through this last year. Quite a few kids who'd previously loved school were coming out crying and not wanting to go in in the morning (not sleeping, vomiting with anxiety etc). It settled down after a week or so, as far as I remember. I think it was partly that the previous teacher had been absolutely lovely, so it was a bit of a shock, and also it was a big step up in terms of the amount of work vs play. Possibly the teacher started out harsh just to make her mark, I don't know.
Anyway, a few parents complained, and I think the principal had words with the teacher and with the kids, and it all settled down quite quickly. They ended up having an ok year; most of them didn't really warm to the teacher, but they were happy enough.
Absolutely have a word with the school and let them know what's going on, but at the same time, don't stress too much about it, give it a few days and see what happens. If it's still an issue in another week, talk to school again, and obviously if the teacher is really crossing a line (nasty comments, threatening the kids with excessive punishments etc), report that and keep a record of it.