I understand that he's a rather touchy subject, and that what he did was brutal. Moreover, I understand that high school students are young. However, when it comes to education, I don't think there should be any boundaries, and I think they're old enough to handle it as long as a responsible teacher is in control. Student's might be able to learn a lot from studying him, and that would at least be something good to come out of what happened.
The Charles Manson case is a well known and unique case in that he was found guilty of murder without actually taking part in it. It should be considered because it provides an example of how mind control and influence and "ordering hits" is considered murder even if the person isn't directly involved.
I think that; if you expose kids to subject matter like this, it will help them understand what law enforcement have to deal with. Therefore, helping them see if they could be capable of doing a job in this field of study will be suiting for them or not. I firmly, believe that kids see the criminology, behind the elite killers like Charles Manson.
Even though Manson killed fewer people than most people believe (he had his group of women disciples do a lot of the actual killing) he was a criminal still, and his crimes could be interesting in the setting of a criminology course, where people are training to understand and prevent these things.
I assume it would make sense for high school students in a criminology course to consider the Charles Manson murders as part of their curriculum. In my experience my high school didn't offer any criminology classes, so the idea of that being a possibility is new to me, but I suppose that case would be appropriate.