Make sure that the only things that they hear is bad things. When their child had a good day or did something outstanding, give them a call to tell them!
They know their own kid better than anybody so there is a good chance that whatever you are dealing with, the parents are too. Ash them what they are doing and see if you can try that in class
Do NOT get upset with the parent no matter how mad they make you. They might go for the low blows so make sure that you are mentally prepared to take on that challenge.
Whenever you are in doubt of what to do, just ask to reschedule the call to a later time adn remember to stay calm when doing this. This will give you a break and them a chance to collect themselves.