Ootp happens supposedly in 1995 and she says she had been teaching for 39 and that means she was teaching from the late 50' (1956). But even so I still don't understand where my mistake is since at no point did I say McGonnagall had been Vodemort's teacher. Which means that how long she has been teaching is irrelevant here. Could you please clarify the error because I have been really looking and haven't found it.

if you mean because of this part:

"Sirius sighed and ran a hand through his face, “You know about the legend?” Harry nodded. “About fifty years ago it was rumored that the Chamber had been opened. There were attacks on Muggle-borns and one died. The school almost closed. But then someone was caught and expelled after that girl’s death and no one else was attacked. I personally don’t think they caught the right person. Nor does Dumbledore. I think the real culprit just stopped because it got too risky.” "

“Who was expelled?”

The fifty years come straight from book 2 and I never meant to allude that Dumbledore was Headmaster then, just that he didn't believe Hagrid was responsible and helped him. They said the same in the book.

Please  I would like  to see if it's viable for me to correct the error you found.

Thanks.