In the original 360 version, you had to do this during the first part of year 7, from the tent on the hill where you use the map - if you went to year 8, you had to start a new game to get this one. The good news is, that has been fixed!
The first chance to get it is after you escape the Ministry of Magic by running from the dementors during the chapter 7 story. The change for the Collection version is that now it also seems to come naturally while playing the story for year 8. After beating the level "Back to School", when you're following the ghost to the next level, you should end up in the lobby and the achievement should pop before you go up the stairs on the right.