
Hi BB,
This is sadly a common situation that I used to see frequently when I used to use beaches and public parks with my dogs. I don't do this anymore as the GP always ruin my and my dogs walks, so we have relocated out of choice.
But you should be able to use a public area without being harrassed. Alas some idiots will always spoil it.
For now, I would keep Bert on a lead. Yes, I know that you shouldn't have to, but until you're sure of his responses it is better for his and your sake to be safe rather than sorry.
If he displays the same response again, I would distract him and ignore his behaviour. Don't make a big issue out of it. Don't avoid youths just keep doing what you always do, but with his lead on.
I can understand your anger and believe me, there have been times when I have lost control of my temper and verbally abused people for such inane, thoughtless behaviour. Sadly, it probably doesn't educate them!
Alas there doesn't generally seem to be any canine respect courses that are run in schools. I do believe that parents should do this though, I have taught mine and she wouldn't dream of going near a strange dog without my and the owners permission first.
Sorry I can't be of more help. Just a shame that people/kids aren't as well trained and behaved as some of our canine friends :(