From what I have gathered Chespin has done nothing wrong in this situation other then punishing you for a misunderstanding. The fact the nade was thrown inside a T room where it had the least chance to hurt an innocent means that rule 2.6 really does not apply here as his intentions were to hurt the people in the T room and he directed that nade into the T room. For that reason this is being:
If I were you I would make a punishment appeal to get your punishment removed as I feel like you just misunderstood the rule in this instance.
Lock.