I went through the PetSmart academy. They just send you to another PetSmart salon where a manager who wants extra money teaches you how to groom. You either get lucky and get a good manager, or you don't, like me, and end up with a catty manager with too many girls in the class and not enough dogs to actually get experience on. I left the 4 weeks academy having never touched a poodle, cocker, or scottie. The next 8 weeks I groomed "under" our manager, where I got dogs scheduled but groomed them while she was doing her own grooming, and she had more interest in grooming her dogs and leaving than helping me at all.Places like that are only as good as their employees and with that being said it's going to be completely random from store to store.
I know with Petsmart they will actually send you to a grooming school, you do not have to pay for it but you sign a 2 year contract with the company.
With Petco you learn from "experienced" groomers but the person training you may have 10 or 15 years of experience or they might only have 2 or 3, so it really depends and you sign a 1 year contract through Petco.
I'm working at Petco and I enjoy it, i have a very experienced person training me and constantly learning new things daily. I started out as a bather and was eventually offered a grooming position.
I still think going to a school yourself is much better. I wish there was a decent one in the area I live in but unfortunately there is not.