My son went to two years of preschool and one year of PreK (summer birthday and we wanted him to wait a year) and never had any problems. Never. The only time he wasn't on green was when he was goofing around in the bathroom or too loud in the hallway. Never because there was a fighting problem. I think he was one yellow 2 or 3 times.
This year he went to kindergarten, so second year at this school. He was on red twice in October. I was horrified!!! He had never done anything like this. Turns out he had a hard time adjusting to new teacher, she taught 1st grade last year and wasn't nearly as patient with the kids as his PreK teacher was, it was a bigger class (11 last year, 20 this year), etc. Also, the boy he had conflicts with also went to his daycare center after school. It took awhile to connect the dots, so to speak. (His teacher was really, really bad at talking to us about things. We had to always seek her out.) It wasn't until March that we realized it was this particular boy he had a personality clash with. He might not even have had so much trouble with him if they weren't in the same class AND going to the after school program together.
My point in telling you me very long story is that it might not be as simple as, he's a mean boy. Hitting, yes, that needs to be reported. Spitting and hair pulling could be her interpretation. Maybe he's just goofing around. I realize I sounds kind of naive or clueless, but he's a boy. He might be use to goofing around that way. Many boys are. It's true that he needs to learn that that behavior is not ok, but it's not really the same thing as "mean boy."
We told our son to play with other kids. This didn't always work, as he and this boy did enjoy playing together. The daycare teacher said they loved playing together and she had trouble separating them. She also said, neither child was innocent and neither child was completely to blame. Sounds about right to me.
Encourage her to avoid the boy. Encourage her to play with other kids. Remember that when school starts, the situation will be very different. School is not the same as summer day camp. The structure is different, and the discipline is different. You can mention this to the teacher in August, but I really wouldn't worry too much. This is not the type of thing that goes unnoticed as much at school.
And don't assume he's a "mean boy" and she's completely innocent. It's possible that she has said or done things that have hurt him or confused him. Remember, at this age the teacher's motto is, "If you promise to believe only half of what they tell you, I will promise to only believe half of what they tell me." She is probably telling you exactly what she perceives to be the truth, but her perception is that of a very young child. Listen to what the teachers tell you they've observed.