Quoting Keyboard Warrior.:" I don't think a small child throwing a rock would bruise like that. I'd ask the teacher/staff. They ... [snip!] ... going on. ETA: Yes, I 'd be upset. Either they know and didn't notify you, or they aren't paying attention enough to know."
That's my thoughts. I'm willing to bet that it's one of those things where they weren't paying attention. I really think her class size is too big for the teacher. I've been wanting to pull her out mostly because we moved slightly further away and the commute sucks, I only had her in because she was in the class with one of her friends from my playgroup but she's since told me a bunch of times that that friend is mean and doesn't like her.
ETA: Above mentioned "mean" friend wasn't at school last week.