My aggressive puppy

Question
I got my puppy 5 months ago. He is a lab mix.  We have been attending obedience classes.  The trainer of the class has brought it to my attention that if this dog was taken to a pound or humane society he probably would not have passed the tests they put puppies through to see if they would be suitable in a home.  I am the dogs primary care giver.  He seems to be learning slowly but still learning.  However, this past week he has tried to bite me twice.  Once while I was loving him and the second time was when I was taking him off his lead outside.  I hardly ever use the lead but sometimes I need to when I have other things going on and he needs to go to the bathroom.  This hasn't been his first offenses either.  He is uncontrollable around children and will hide under the car and snap at me when I try to get him to come out.  He hates being left alone and I try to take him wherever and whenever I can.  He is getting bigger and I am afraid that I will soon no longer be able to control him.  I was also very upset when he did try to bite me because I give him everything.  We were given a collar that has the two prongs on either side of his neck to act as if his mother were correcting him and that seems to be working when we walk him.  But I don't know what to do about his aggressive behavior.  I am writing you because I was wondering what my options are.  People have suggested sending him to people that take a dog for a while and train him and then send him back. But does that work?  Am I not giving him enough time?  I'm even unsure of the correct question to ask.  Hopefully you will have some insight.  Thank you for your time.

Answer
I am much better at properly socializing young puppies to avoid such problems, than fixing them.  You don't say how old he was when you got him, or how old now.  If he is well over 5 months, he will not get much bigger.  However, the older he is, the longer it is going to take for you to take over as top dog.  The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  I hope the instructor in your class is making that point.  If not, drop out if you can still get your money back.  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

The other thing, who adjusted the collar?  If not adjusted right, the prong collars can injure the dog.  Still, you may want to switch to a head collar.  The leading brands are Promise,  Haltie, and Gentle Leader.  They have a strap going around the dogs nose looking something like a muzzle.  They work by pulling the dogs head around.  No other way gives you such great control with so little force.  

You might contact a Lab rescue.  These are dogs that lost their home, but were taken into a foster home to be retrained as necessary and placed in the right home for them.   You may find a rescue near you starting at
http://www.akc.org/breeds/rescue.cfm  Rescues are overloaded, and likely wouldn't take him.  I suggest asking them to help you work with him.  They have experience with fixing troubled dogs.  They would love to see you be able to keep him.