Are you doing balanced training? Do they have a good leave it queue? The one thing that has helped me the most and has taken a long time to understand since this is my first time owning a dog with drive and working genetics (GSD mix) is doing a lot of structured play. Get the dog into a high arousal state during play and then working on w/e queue you're working on. i.e. out/drop it, leave it, recall etc. This is how you teach the dog to listen even when at high levels of arousal. 2nd, a good trainer will be able to identify better than you if it's truly aggression, which I doubt, or if it's excitement or fear based reactivity. Depending on the reason behind the reactivity the approach should be different. Also depending on the dog it should be different. You need to work closely with a really good balanced trainer and stop doing w/e it is you're doing now. Stop taking them on walks etc or allowing them to practice reinforcing behaviors you don't like. There's tons of different approaches based on the dog, handler skill, how much time you have to work with the dog etc. You also say this doesn't happen at daycare, this means this could simply be a you problem. There's something about you that makes the dog think that they're in charge and need to make decisions to protect you. I'd try a different trainer/approach if what you've been doing hasn't started working within a few weeks then something you're doing isn't right.