Is it pretty much a given that pure positive Obdnce classes will kick out aggressive dogs?
Not necessarily. It depends on the skillset of the trainer and also how well set up the facility is to accomodate such dogs.
When we had Cassidy years ago, in her first class (we did not get her until she was 20 weeks old and started an obedience class a few days later), there only 6 dogs enrolled, including her. Often 1 or 2 other dogs weren't there, so we had a trainer and one or two assistants in a large room with very few dogs. She was scary smart and did quite well in class, but it was clear that she was already leash reactive.
It was not usually a problem because we had lots of distance from the other dogs in the class and could face her away from them while we worked, but in her next class (same trainer, same location) there were twice as many dogs, and it was very difficult - stressful for us, stressful for Cassidy, and also highly disruptive for everyone else in the class. The trainer had previously worked at the Marin Humane Society, assisting with classes before going out on her own. MHS had recently started doing "Difficult Dog" classes for reactive dogs. About 2/3 of the way through the course we were taking she suggested that we might want to drop out of her class and take a DD class at MHS, which were being taught by Trish King, who ran the Behavior and Training department there and was also our trainer's mentor.
We could have stayed and finished out the last few weeks if we wanted to, but frankly we were relieved, and the DD class turned out to be great, a much better and less stressful environment for all of us and we learned a lot. That trainer BTW, has a private training business specializing in aggressive dogs, so she was definitely capable of working with us and our leash reactive girl.
The last class I took at MHS was with Halo a year ago, and it was Family Dog 2. In that class there was a highly leash reactive dog who spent the entire 6 weeks inside a "condo" of barriers to block his view. The trainer for our class also taught DD classes, so she was equipped to deal with this dog, and the facility was too. Keefer's last class there a couple of years ago had a reactive dog too, although not as bad as the one in Halo's class, and they just set up barriers to block the view of dogs on either side, and whenever we were working in the middle of the room, doing recalls and such, they'd put a barrier in front too.
It all depends on the facility and their policies, but it can work if there's a competent, experienced trainer on board who is willing to work with you. Personally though, if he's really bad I'd look for a special class for reactive dogs. You'll probably get a lot more out if a class that's designed to tackle the exact problems you're having.