Canada high school legal self defense?

Breaking bones likely won't be seen as defense. Defense is doing what you need to do to get away and out of the situation. Walking away does that. You only fight back if you HAVE to. Just getting shoved? You;d go down hard for really hurting someone if they only had pushed you.

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bring yoursekf a trekking pole to school, crack his skull with it or just hit him in the limbs, your choice

You have the right to use the amount of force NECESSARY to stop further attack against you. If you TRY to hurt them, you have gone past self defense. If you happen to hurt them, it may or may not be self defense.

No - self defence is literally just to defend yourself from harm. If someone punches you and you punch them back, you are simply returning the attack.

However, if there attack causes them harm thats a whole different matter. For example, if they swing a punch at you and you move out of the way which results in them hitting a wall, its entirely their own fault.

you only have the right use the amount of force necessary to defend yourself which means you can not continue to beat them after they are no longer a threat to you.