you should have been called ASAP!!!!!!!!!!!!! you need to make this very clear when you have a meeting with the principal.

At my school, any time a student gets a blow to the head (fortunately ours are usually from bonking into the monkey bars or smacking into someone on the soccer field, not being pummeled in a fight) the parents are called and the student is given a note to take home. There's no excuse for a student coming home bruised and swollen with no notification to the parents.

The school is probably not able to give the kind of detail about what's going to happen to the other child that the previous poster suggests. Students have privacy rights, even the ones who attack other kids. But the district should have a clear and publicly-available policy outlining the specific consequences for such an offense, and the parents of the injured student can certainly insist on confirmation that this policy was followed.

If this is your child, I would call the district office. If they have a "risk management" department, that would be a good place to start, since they're the folks worried about insurance and liability. If they don't have such a department, go straight to the superintendent's office.

Is this your child or someone else's child? The title of the post suggests it's your child but the post itself suggests it's not.
Whosever child it was, the parents should have been called by the school, of course. Head injuries in particular deserve a 'heads up' given to parents. Parents should be warned so they can observe the child for signs of a possible concussion.
But why didn't the parent ask about the bruise on the cheek? Or the goose eggs on the head? Regardless, schools open themselves up to lawsuits and don't live up to what's appropriate if they don't notify parents of any injury that occurred on school property during school hours.
If a parent is called, they thank the school for calling but as this injury was the result of a physical assault, the school should also be explaining how this is being handled. 1 - how the school will make certain sure this never happens again but explaining how it happened and what they'll do differently from now on to make sure it doesn't happen again. And 2 - what will happen to the other child who assaulted their child.
A punishment is hardly all that should happen here - why did this other child hit? Does the child have a history of doing this? Was this a first? What led to the assault?
If it were my child who'd been hit, those would be my questions - telling me the child had been 'punished' tells me nothing and doesn't reassure me in the least that this won't happen again.