What happened to you has happened to all of us to some extent. Actually what was written was tame compared to other situations I have experienced as well as others.
In my area this would be a state issue as sportsmanship is something the state can deal with and could bring suspensions for behavior and certainly behavior immediately after the game. These individuals are representatives of their schools after all. Nothing wrong with letting the assignor know, but I would also report this to the governing body if they have a policy in place for these kinds of things.
You are right your partner should not have engaged the fans. I would disagree that you never engage them if they are threatening you personally or they are invading your space. You might have to say something to them to rectify the situation. Or what happens a lot are people will come up to you and say, "Nice game." Be respectful to those that are respectful to you and most fans can be nice.
Another thing you can do is find ways to leave the gym in ways where you can avoid the congestion of fans. That is not always easy to do, but you can make sure you leave in a way that you can avoid most confrontation. Sometimes try not to make real eye contact with people while leaving and when you are ready to leave the locker room, you are prepared to leave the building.
Look at it this way, it could have been worse. You just had words with a stupid individual that has little perspective on what youth sports is all about. That will be something you will witness several times over your career if you stick around. Then you will laugh about these things and know how to make them feel dumber than they are by your behavior.
Peace