1. I registered for the site before I joined a credit union, selecting the credit union I intended to join. I then joined that credit union. Each credit union is different, but the one I joined only required a minimum $25 deposit. You can change credit unions after you register. But you should be a member of one to get an authorization number. You will be able to look around the site once registered. I suggest reading the FAQ.
2. Have no knowledge of the teachers discount. Once you are registered for the credit union program, you apply for an authorization number when you intend to buy a car. The authorization number, once issued, is good for six months. You are not issued a number automatically on registering.
3. Note that while the 2015 Corvette cannot be priced out on the site as of today, it is an eligible vehicle. I think it cannot be priced out because Chevrolet has not yet updated the "build and price" for the 2015 Corvette on its web site, so it is not available on the mygmdiscount.com site either.
4. Dealers are not required to participate in this program. Nor are they required to make all models eligible for the program. However, at least one of our sponsoring dealers has posted on this forum site that they will honor supplier and employee pricing on orders for all models of 2015 Stingrays, (this may not include the Z06. Doesn't say), so they would, I think, honor the credit union price, which is the same as the supplier price.
If I have gotten anything wrong, I am sure someone will correct me.
5. When you buy through this program, you cannot use some GM card discounts, such as the old Grandfathered program that I participate in. Again, this is in the FAQ.