You absolutely did the right thing Junkyard. Schools these days implement policies of total disempowerment, indoctrinating kids (including myself) with this bullshit idea that defending yourself is wrong and somehow worthy of punishment (ie. detention). I went through school getting bullied to hell. I was a small skinny kid, and an easy target. The asshole school policy makers kept telling me to seek help from teachers and counselors, who would just sit me and the bully down and make us shake hands after a touching conversation and be "best friends", at which point we'd leave the office and he'd say "I'm gonna fucking get you after school". That or they give out weak 2 day suspensions. Point is, kids these days aren't afraid of any consequence for their actions because there aren't any with all the legal restrictions. In fact I'd go so far as to say I agree with bringing back corporal punishment, just not used in the excessive amounts of back in the day.

Your kid learned a valuable lesson from you, that he can be an empowered and strong individual with self respect. Kids who are brainwashed into not defending themselves, not standing up for what's right against bullies, are the ones who get incredibly low self esteem, depression, anxiety, often turn to hard drugs, and every so often snap and go on a school shooting because it's the only way to stop being a victim that they know of. When I was in highschool I never would have gone on a school shooting, but I didn't have the mental image of an "evil" kid for shooting the place up, I could understand how daily torment could push someone to do that. There's only so much mental strain the brain can take before it breaks and defense mode goes into overdrive. We need every kid in the system who can prove that you don't have to be either a bully, or a victim.