My son is also in kindergarten and was being attacked by another kid about a month back. He is autistic and nearly non-verbal, so I knew he'd get bullied; I just expected it to happen a couple of years from now rather than 2 months into kindergarten.

We weren't even notified of the first instance of the bullying. When he got home on that day, my son used his limited language to tell us that a boy hit him. We were eventually able to get the boy's name from him and contacted his teacher who told us that the boy bumped into my son while they were walking in a line. The teacher said she'd keep them separated and keep an eye on them. We couldn't get any more information from my son, so we accepted that it was an accident. The very next day we got a call during the day that the same boy walked up to my son at lunch and hit him, but they were nonspecific about how. When he got home we saw that he had a busted lip from this altercation and my son said he was hit in the nose and mouth. We sent an (admittedly angry) email to the teacher to get everything on record and she said they didn't see the busted lip. She was going to contact the boy's parents and have a conference with them. That seems to have worked since, as far as I know, there have been no further attacks on my son, but I can't say that it isn't a constant worry in the back of my mind.

All of that is a really long winded way of saying: We let the school's attempts to resolve it take their course and it appears to have been effective quickly. Had it not been effective we would have actively hassled them until we were satisfied.