I second the "no jumping to conclusions, please."

Just as conformation dogs are bred for their looks and temperment, guide dogs are bred for their temperment and ability to preform. If I had not been educated and gone through the process of raising a guide dog myself, I would probably think the same as you.
And just because your teacher's dog still pulls on the leash and can't handle pep rallies does not mean he won't become a guide dog -- it just might take a lot more time than it usually does. My Abby-girl's brother took a year to be able to go into training with a blind person, and even then the match wasn't right so he's back in holding. My Abby was a puller (probably something to do with my lack of training skills at that time) and she graduated basic and advanced training and went into holding within 3 months. That's faster than most dogs.
I have never EVER met a lab as calm as my Labby-Abby. But when you wanted her to be up and working, she would. When you wanted her to be "off," she would. You can't get that from a dog with baggage; you can't get that from a dog of indiscriminate breeding.

Oh and to the person who asked the original question: lots of research is needed to decide whether or not you're the kind of person who can handle raising a guide dog. It's a huge emotional investment, and just as the dog is getting to be a nice dog, not a puppy terror, you have to give him away.
It's such a rewarding expierience and I can't wait to do it again -- right now I'm focusing on my school, but as soon as I get on my feet and out of high school and college, you can bet I'll have another GDIT.