Ann - i'm sorry you got that call. I'm not going to be much help because I don't have much personal experience with hitting, except I know when we went to early childhood classes, this was very normal behavior, and the following book was recommended:
Hands Are Not For Hitting Book

DS4 did hit a couple times when he was really frustrated with something, and we gave him immediate consequences (no warning) that could not be earned back (loss of bedtime books). That worked for us. Good luck!