These are the things that happened. I'm confused like others as to what the trainer thought you could have done. Unless your horse is a known kicker and you weren't taking proper precautions, it is nothing to be ashamed of. I had a mare I knew kicked, and when we were lining up for the entrance to a flat class I always made a point of keeping her till the very last and she had a braided tail with the biggest red bow. But a class letting out had a ton of kids and ponies and they all came cantering out, and slammed on the breaks in the isle, leaving a tricky situation to get in the ring. I literally was yelling " my horse kicks please move so you don't get hurt ". Sure they moved, behind me... we got bumped and jostled and yes a kid got kicked square in the thigh because she backed her horse up into mine cause they were more concerned about pictures. I felt bad that she was hurt, but common sense says clear the gates before you stop and if someone says their horse kicks get the heck out of the way.
You can't help that horses have natural reactions. Now you just know for next time to keep your horse a little off to the side (or at least the firing end away). You've apologized, there's nothing more you can do. Deep breath, hold your head high next time. And if you see any less than brilliant people lingering to close to your horses rear, just let them know he has kicked and it's not smart to walk behind a horse.