on best behavior
on (one's) best behavior
Being very polite or exercising exceptionally good manners. Primarily heard in US. Now, you kids must be on your best behavior for your Auntie this weekend! Good morning, class. I hope everyone was on their best behavior for the substitute teacher yesterday.
on one's best behavior
being as polite as possible. When we went out, the children were on their best behavior. I try to be on my best behavior all the time.