Some more suggestions:
"Clean up one's act" i.e. I used to skip classes and party up all night, but I have since cleaned up my act after my parents' ultimatum of staying in school or ending up in juvenile detention.
"Turn over a new leaf" i.e. It looks like Bob has turned a new leaf from his drug-addiction days.
These two idioms do have the same meaning of improving one's bad/unacceptable behavior(s) from the past and starting to behave more responsibly/sensibly and to become a better person......
- For Teachers