He can't tell off his students for fighting dirty when he's the one that taught them to fight like that and do whatever they can to win. If he had taught them properly they wouldn't have acted that way. Neither of those kids would've done anything like that before starting trainingJust a couple of thoughts on this whole thing.
Loved the series, thought it had reasonable morals, johnny started to pull his life together (resilience), Daniel showed some compassion helping Robbie at the tournament, Sam (and moon, in a way) showed that kinship is more important than popularity.
And Miguel stood up to the school bullies.
Later on, Johnny is trying to make good, admonishing both Miguel and Hawk for playing dirty.
On a general note, if your coach isn't pushing you, you're not going to be your best. Which is why in the real world, very few successful athletes are coached by friends or family.