I'd start tonight by retiring that flexi leash and getting one with a bit more control - or keep the flexi locked in place, keeping your dog within 4 ft of you at all times. There is no need for your dog to not be next to you while walking (unless it's a remote area, beach, etc where you want some extra sniffing space)
And yes, I think it's time for some obedience classes - it's a great class for both the puppers to learn some manners AND you will learn a lot as well. Highly recommend you look for a class/trainer that promotes positive reinforcement.
The great news is your pup is still young and there is lots of time to get this all under control