Never scold a dog that has come to you when called, no matter how mad you are at something he has done. When giving a command to a dog such as come, sit, and stay. Say their name and the command one time, and one time only, before making them follow through with what you asked. If you say the command two, three, four times then the dog gets conditioned to hearing it several times before having to act on the command. They will actually wait to hear it several times before reacting.
Dogs learn quickly, even bad habits.
These can be very helpful. There are several advantages to the class situation as opposed to private lessons. Financially they are far more affordable and the exposure to other people and dogs can be very beneficial.