clean slate

[clean slate] {n. phr.} A record of nothing but good conduct, without any errors or bad deeds; past acts that are all good without any bad ones.

Johnny was sent to the principal for whispering. He had a clean slate so the principal did not punish him.

Mary stayed after school for a week, and after that the teacher let her off with a clean slate.

Compare: TURN OVER A NEW LEAF.