A couple things I've noticed in the numerous classes I've taken over the last few years: In a new environment - new dogs, and place they've never been before, my dogs have tended to be much more distracted in the first class or two, and then gradually got better as they adjusted to the routine and it all became more familiar. Also, the beginning of class is more distracting because everybody is coming in around the same time and it's noisy and hectic. Once all the dogs and people are settled in their spots and class begins, it gets better. You just have to get past that initial rush of excitement, and it really helps to be prepared for it in advance with a plan.
My dogs do very well in class when we're actively working on training a skill. When I lose them it's usually either before class starts, or between exercises when the trainer is describing and maybe demonstrating what we're going to do. They start looking around and noticing all those other dogs they'd rather be playing with than boring old mom! If your dog is highly distracted, try bringing a kong or other stuffable toy that you can give her at those times where you know she'll have trouble focusing on you. I like to use spray can cheese because my dogs love it, and I can spray a little inside a toy, and it doesn't take long for them to lick it all out. If there's too much good stuff in there, she may be more interested in working on her kong than coming back to you when it's time to start working.
I also teach all my dogs to "find it" from the moment they come home. What it means is that there's yummy food on the floor for them to sniff out. It's handy in all sorts of circumstances - I've used it as part of a recall game, to call my dogs off chasing the cats, if they get outside and don't want to come back in - and it's really easy to teach. I start with a bunch of really tiny soft treats, the size of a pea or less. Cut up Natural Balance rolls, jerkey treats, cheese cubes, or bits of turkey dogs are great to use. I drop a treat on the floor and tell them "find it". The second they get to the treat I mark it, either verbally "yes!" or with a clicker. Do it over and over again, for a few minutes several times a day, and she should immediately get the treat and then look back at you. Start tossing the treat further and further away so she has to go look for it and then come back to you. When you can call out "find it!" and she comes running from another room in the house, you'll know she's got it. (Very handy when you drop food on the kitchen floor while making dinner. 
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When a dog is really distracted they may be so focused on interesting sights and smells that they don't really "hear" you, and at those moments I'll toss treats AT the dog. The tactile sensation will usually get their attention, and if you toss a handful they'll bounce off all over the place, occupying her for a few seconds while she scarfs them up. I used this technique all the time with Keefer who was a very distractable 13 month old in his last class. He'd look over at another dog, start up with his yippy "let's PLAY!" bark, I'd say "find it" and toss a bunch of treats at him. He'd immediately break off the other dog, dive on the treats, then look back at me. I could then mark and treat his attention, along with happy, enthusiastic praise. I really like using this technique rather than corrections because there are never negative consequences for being around other dogs, which can make a reactive dog even more reactive. Other dogs should always mean "Yippee! Where's my treat?"
He also tended to be just as distracted, if not more so, by a single dog outside class then a whole class full of dogs, so I wouldn't worry about that too much. Putting as much distance between her and the other dogs in class, and facing her away from them works well, if you can snag a corner spot that's ideal. Also, do as much attention work with her in the meantime and between classes as you can, around the house, and outdoors where there will be some distractions, but nothing too major. In a place where she might see other dogs but at a distance is perfect if you can set it up, just close enough for her to notice them, but far enough that she won't bark and lunge. If you're on a walk and see another dog coming towards you, do an abrupt about face and go the other direction. It helps to work on this outside of class too, come up with a verbal command to signal a rapid turn (I use "this way!";), then stick a treat in front of her nose to lure her around at first, then mark and reward. ALWAYS bring treats!
With Keefer, if I saw something in the distance he might react to, I'd stop at a safe distance and wait for him to notice it. Eventually I knew he'd look at me, and the second he did he got a jack pot of treats, give one small piece at a time with effusive praise so she's being rewarded for several seconds. The more you do this - no command, simply wait for, then mark and reward her spontaneous attention, the more she'll start paying attention to you and the less she'll react to distractions. But it will take some time, so you have to be patient. As she becomes more comfortable in the presence of other dogs, gradually work her closer and closer to them. In just a couple of weeks I was able to go from Keefer going off because of a dog across the street, to being 3 feet from a dog behind a chain link fence right next to the sidewalk. Even when the other dog went from standing there calmly looking at him to barking and jumping at the fence he didn't bark back. I stayed calm and waited, and sure enough he turned and looked back at me. BIG rewards!!!!
Since your dog is just a young puppy with a short attention span this may not be a big deal to fix. Cassidy was HIGHLY leash reactive, and we worked on it for over a year. While she got much better, she was still a work in progress. With her it was fear based, she had weak nerves and was okay with most other dogs off leash, but horrid on leash when her options were taken away. With Keefer it's more about him learning to control his impulses, which is why he's getting the idea so much faster. He's extremely social, and not at all aggressive towards other dogs, he's just a butthead boy going through a phase and I'd let his training go.
Good luck, RELAX, and have fun!