Do I have a case if my son was sexually harassed and assaulted at school by another student?

During the 2014-2015 school year a 3rd grade boy was talking about porn to my 3rd grade son, asking to play games to see his penis, was watching him urinate in the bathroom at school, encouraging him to do things like urinate on the playground so he could see his penis, etc. He was also harrassing my 5th grade son, punching and hitting him when he saw him and swearing at him. We contacted the school and I told them that this child had a strange attraction to my children and I did not want them alone in the bathroom together. On the last day of school in 2015, the other student pulled my student into an empty classroom, pulled down his pants and said he was going to "kiss his penis". My son pulled his pants back up and left the room and told a teacher. We were not called or contacted. When my son got home, he told us what happened. We called the police and filed a report. The school never contacted us and the next year the child was not at the school (the parents transferred him).