A slap on the wrist
Definition 1 of 2
Pro Tip 1/3Not Literal
The phrase doesn't involve actual physical contact; it describes a non-physical, light punishment. 
Though cheating is serious, he got a slap on the wrist instead of expulsion.
Pro Tip 2/3Figurative Warning
It can mean a mere warning to signal disapproval without severe consequences. 
She was late to work and received a slap on the wrist from her boss.
Pro Tip 3/3Expectation Violation
Used when an action deserves more punishment than what is given. 
Many believed the corrupt official would face jail time but all he got was a slap on the wrist.