aggressive dog training

I have a 5 month old male German Shepherd dog in obedience training. It has been aggressive! Help?
Last Thursday we went to class and in the class of dogs are not allowed to associate with others. Not allowed to sniff each other or do things connected with another dog. My men, started barking and lunging against the other dogs, he has never done before. I really feel that only wanted to play but when he acted as the others jumped back and pulled back to their dogs, which made him worse. Now, after our walks, it is housed in the heel, not paying attention, and is acting very aggressive when we are and welcome to our walks. Tonights walk finally had to almost sit on it to leave this dog alone. It was well before the class last Thursday. Let dog parks, pet stores and walk 2 times a day. That usually goes everywhere I go, weather permitting! What now? He will be more than 100 PDS and we are using Martingale collar. Any suggestions?
Watch some episodes of The Dog Whisperer. Sounds stupid, but he makes very good points. The behavior of a large number of dogs may be unsuitable due to its struggle to establish himself as leader of the pack. If you establish yourself as the alpha dog will look to you for acceptable behavior and no action.
Aggressive dog training and aggression rehab tips (www.k9-1.com)