I don't think leash pops are the best training technique, especially for a puppy. It sounds like your pup is just not ready to be trained in this environment yet. You are dealing with too many distractions, and you need to take a couple steps back and work on training for focus (watch me) in a quieter situation until the command is more solid.
A leash pull or pop is a punishment, even though it probably doesn't hurt the dog, it is not a pleasant thing. So if you are in a class that you want your puppy to enjoy and come away with it being a positive experience, a leash pop is not a great idea. Your trainer is right, training focus does take time, and you may need to just work on it more at home in a quiet room with no distractions, then move it out to the backyard where there are more distractions, then move it to a parking lot or somewhere with more distractions, then a park where there might be other dogs, etc... And then maybe you will be ready to deal with the level of distractions that are present in a class.
Some people use a technique to teach focus where they hold a treat in their mouth (like a piece of hot dog or something) and spit the treat at the dog when he looks at them. I've never tried this method, maybe some others will have suggestions too.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old