In that sentence it isn’t very clear indeed. Only context could give you that information.

Himself/herself or themselves is translated by ‘se’ in french. There is no distinction

You could emphasize the fact that they are hitting each other by saying:

… pour taper sur les autres (to hit the others)

But in that case it is not necessary as one could easily understand that one pupil isn’t gonna hit himself voluntarily

As mentionned in some answers above, you need a context to know exactly what this means. As I recall many scenes from the movie it seems to me that the right meaning is: “The pupils of Smeltings also had sticks which they used to hit each others when the teachers couldn’t see them”.

I can imagine this easily, as the teachers are distracted by something else or are out for a moment the class would become messy as they begin to hit each others. These students from Boudlar are two shabby to hit themselves (so it seems to me, and that’s why the full context is required to translate this correctly).