I go back to the strict religious practices and beliefs. Could this young man have done something that his murderer thought justified this punishment? Had he "ruined" a girl or anything like that? I'm not trying to smear this boys name or memory, but this thought comes to mind. Fifteen is an age of raging hormones. Or, by orthodox law, what acts would be punishable by death that fit in the realm of possibility with this young boy? I think the religion is the key, and that this was punishment for some perceived and unforgivable deed? Or, had something been done to him that he was being silenced for, or that he had witnessed?
The more I look into it the less I feel this is the case. According to the classmate, all the older boys were home with their families so only the younger boys were at the school at that time.
Judaism, in general, isn't an anger filled religion. There is nothing Chaim could have done that would have been worse than murder in the eyes of Orthodox Judaism. Even if he was having relations (heterosexual or homosexual), there is no sect of that religion (no matter how esoteric) that would have allowed murder, especially on Shabbot when this crime was committed.
Another thing, this was for the most part an organized murder. The killer did not make any noise or go into a frenzy, which shows a certain degree of intelligence or experience. I believe this eliminates the classmate theories. In theory, could it have been a teacher? Yes, but this was a brutal crime and it was unlikely a teacher could have done it without causing suspicion in such a closed environment (much less never do something like this before or after).
I believe this was a killer with some degree of experience and self-awareness (who knew very well what he was doing was wrong). Self-aware enough to sneak back into the room and move the body (for whatever reason) and still not get caught.
If he was a stranger (which is what I believe) then the #1 reason for this particular murder would be antisemitism since why else would he chose this school? There did not seem to be much overkill. Just a few stabs to the head. Usually overkill to the face/neck is the sign of a very personal murder (ex. Nicole Simpson, Lizzie Borden). If this was a homosexual crime or a jealousy crime, we would expect to see some form of overkill. Rather this was quick and apparently painless (Chaim was asleep and there were no signs of struggle). The killer did his job and left.
If students/faculty had ANYTHING to do with it, then they would have hired a hitman since this was a crime done with precision. What I believe rules that out is that the UNSUB left something or at the very least felt he made a mistake, then came back to the body 45-60 minutes later. This would be a very inexperienced hitman. Plus I'd think very few professional hitman would agree to murder a boy.
The murderer would have to be agile enough to sneak in two times and be quiet enough to wake up nobody. I would do research on known antisemitic individuals who lived in the area at that time who had a violent history and were not too old/crippled. Maybe white power/skinhead groups in the area. Could have been an initiation crime.