I'm sorry, I don't really have any tips to offer as I have the same problem with my Chihuahua. She thinks she owns the whole world, and that it is her duty to protect her territory. On her walks she keeps an eye out for dogs, and checks every house gate to see if there is a dog at home. She has always been like this, even though she's come across so many dogs in her short life, she's never going to grow out of it. However, she has become a bit better. Everytime she barks or tries to pull over to a dog I tell her no and try and face her the other way (which does help her to stop barking). It's hard with a small dog because every other dog that we come across is bigger than Abby so we have no other option but to pick her up (which I think increases her fealing of dominance over the other dog), but seriously if she was to get into a fight (which did happen once, but luckily the other dogs were kind of smallish) she'd get squashed so it's better to be safe than sorry, in my opinion.
I've had people say all kinds of things about Abby when they see her barking in this aggressive way, but they are usually just jokes because of her size. What some people do which annoys me greatly is when Abby is aggressively barking at someone's dogs, and I am trying my best to sort the situation out and take her away from the dogs, and then the owner of the dogs comes over in our direction so it makes everything worse! I don't think they realise that it just makes a bigger problem if their dogs are closer!
But the thing is, Abby gets on SO well with my other dog.
Just keep taking your dog to puppy school, and she should learn to interact better with the other puppies there.