donnarider:

Headcanon that Orion and Walburga occasionally wrote to McGonagall to hear if their son was behaving.

The first few years McGonagall replied listing all incidents and pranks. Until she noticed how defeated and unhappy Sirius always looked when he returned after the holidays. (She could have sworn she saw bruises on his wrists one day).

So she changed tactics. She sent the Blacks detailed letters telling them how much Sirius had changed for the better and how well he was performing. And instead of telling his parents she called Remus into her office and told him everything Sirius had done hoping that he would have a good influence on the boy. This happened quite a lot so that one day when Remus was told to go see McGonagall in her office he just dropped the books he was holding and lifted his arms desperately yelling: “OMG, PADFOOT! WHAT IS IT NOW? WHO DID YOU KILL?”

When McGonagall ran into Sirius and Remus snogging in a deserted corridor in sixth year she nearly lost all hope. Remus was never called into her office again (at least not when it concerned Sirius). Instead someone else had to live with this burden. A very furious Lily Evans who one day climbed into the common room face red, a knive in her hand with James Potter following and trying to hold her back.

“SIRIUS MORON BLACK! JAMES MIGHT HAVE TAKEN MY WAND BUT THERE ARE PLENTY OF MUGGLE WAYS TO KILL YOU! YOU WILL APOLOGISE TO THOSE THIRD YEARS OR SO HELP ME GOD!”