Have you not yet taught your child about 'private' parts of the body? He is eight.
He should know that certain parts of the body are private.
AND he should know that he is to be held accountable for any misbehavior which happens at school. That if MOM SAYS SO, he DOES so.
The fact that you are telling him not to do something ('don't touch a person's private parts') and he is not following directions means it's time to make it loud and clear. I'd start with taking away privileges the next time his teacher mentions this problem to you.
As in - NO TV for a week and NO Video Games for a week.
If those are not things he's usually interested in, what's his favorite thing to do? For my kid, it is Legos. When he breaks a *serious* rule (and not touching people's private parts is a serious rule your son IS old enough to understand unless he has some sort of profound intellectual disability), you take away the thing he loves most, for a week. NO, he may not 'earn it back'. He disobeyed a very simple and reasonable request-- keep your hands off of other people's bottoms.
My son learned pretty early on that we ONLY touch girls/women on their arms or shoulders. Period. Any place that our underwear covers is not to be touched, period. He is six and can figure it out. Don't let your son pretend "I don't know".... you explain private body parts to him. And then, follow up every. single.time you hear he's doing this-- he loses a privilege or a special toy for a significant amount of time. Don't let him make excuses and don't make excuses for him. At some point in time, we tell our kids to do what we tell them to do For Their Own Good, and if they don't, then there is a consequence to ignoring that.
I'm the mom of a boy, but if I had a little girl whose bottom was getting grabbed and rubbed by a classmate, you can bet I would think the parents were in denial if they kept saying "He doesn't understand". You make him understand. That's your job, mom. You MAKE your kid understand.