Oh goodness. Yes, without question. your 8 year old has been molested while at school. I'd tell the teacher, counselor and principal like yesterday. good luck
PS: most kids who molest peers are doing something they learned so it is in the boy's best interest to be found out to offer intervention for what is going on with him.
Report the incident to the school officials, don't throw it out of portion it's a very serious situation and should be dealt as such. Kids do what they see people doing its neither of their fault, but it needs to be addressed