Poo,
It sounds like you have 2 different issues. Here are my opinions on them.
First, the puppy class. If there are uncontrolled large dogs in the class who you already know are aggressive, I'd try to get my money back. Then try it again next session - call in advance and make sure that the one-to-one ratio is going to be enforced next time! I'm one of the biggest advocates of puppy classes on this board, BUT your pup's safety comes first.
Second, what you described with the ball of dirt sounds like object guarding to me. It is a
very common problem in dogs. It's different than normal puppy biting. When you approach a dog who has something he wants to guard, he gets rigid, growls and then tries to bite. Here is an excellent article on the topic (it's how I worked through the problem with Joey):
http://www.clickersolutions.com/arti...ctguarding.htm
Good luck!