Yuck, this is really grim. I would be taking it very seriously and taking action asap. It is not your responsibility to single handedly sort this out or stop it happening. That would be the case even if you were a fully qualified teacher, let alone a trainee. It needs dealing with for a number of reasons:
If these boys were to do this in public, e.g. on a bus or in the park, they would be committing a criminal offence and could be arrested.
There is plenty of evidence that this is not innocent scratching or 'rearranging', but deliberate sexual harrassment.
This type of behaviour can and does escalate if left unchallenged. By reporting it, you can help protect others.
'Acting out' in sexual ways can sometimes be an indication that a child has themselves been exposed to an abusive or inappropriate situation.
I used to teach in Secondary, and although this never happened to me in the classroom, it did happen when I was tutoring and was alone with a student.
It's a long story, background is that he also did it to a more senior member of staff but she did not feel able to take action or even confront it. When it happened to me, I phoned the non-emergency police number to explain what had happened and why I felt uneasy. They invited me in to talk to an officer who said that they weren't going to press charges but they were going to speak to his parents and his school, so I felt relieved that it was being dealt with. I would do the same again- reporting it is absolutely the right thing to do.
Even if your mentor is unhelpful, there will be a Child Protection person in your school (I would hope they've told you who it is!) and that's who I'd go to.