If you haven't already, I would start documenting everything. Just last week didn't this same child have a nasty bruise on their head as well? I remember something about dropping off during bus pickup and she didn't mention it, just silly stuff about taking pics. It may have been the week before last. I have an 18month old DCB who was forever hitting his head for a couple months. One or 2 looked similar to that but usually more of an impact spot in the middle rather than nothing.
Just for liability reasons I would document and what their reasoning was for the injury.

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