Guess I should have said what I'd do if in the same position - and what I DID do when my pup was 6 months old and SO EXCITED ABOUT EVERYONE AND EVERYTHING in group classes....
I arrived early. I let my dog be excited and social for 30+ minutes before asking anything from him.
I only used a buckle collar. This is because a lot of the thicker muscle did not fill in until my dog was closer to 8 months. It was also because I had a very responsive and soft on the lead dog. <= The training that Usually Lurking describes is what I did to get that. Well that and remember treats are not the only reward for good behavior. A lot of people out there get hung up on giving treats and GOOD LUCK in problem solving if you have a dog like my Bertie or my novice A dog.
Novice A dog would not eat anything in public. He would spit everything out. I think nervous stomach. Bertie was not as extreme, but yes - he was not overly food motivated. He still isn't.
In the training ring - I train in conditions where I know he will be successful.
I get that some places might not give you any options, but if you have the ability to steer off as far away from other dogs as possible while working your dog - that's best.
Give your dog a big enough bubble to help them learn to work with distraction. As they are successful, you can move closer to other teams when working.
In the first post - you said the prong was successful and as I said before - if you have a good trainer guiding you and teaching you how to work with your dog and this is a temporary stage and you'll be getting back to using a collar that you can do CGC tests, and whatever beyond in.... go for it.