In Harry Potter and the Deathly Hallows, when did the mistreatment of students begin?

I know that Snape was assigned to Headmaster of Hogwarts prior to the beginning of the school year.





There was also a part in Deathly Hallows about how if a student got detention, they were used by DE's or Slytherins to practise the Cruciatus spell on, making them victims.





I was wondering whether this treatment of the students began at the beginning of the school year or whether it was toward the end?|||Beginning of the school year...remember that some of the Voldemort麓s (Death Eaters) followers were now teachers like Alecto and her brother and since Snape was now the head master they were in total reign...those bastards|||It was definitely in the beginning, because that's when Snape took over. However, they didn't mention it until the end, because that's when Harry came back. He wouldn't have been aware of it otherwise.|||Everyone had to learn the dark arts and they ALL had to take a class about how muggles are filthy. it started at the begining of the year when the death eaters started teaching. cuz they had time to write messages on the walls, rally supporters ,get in troubl,e and get nevilles gran in trouble.


neville ginny and luna were the in charge of all that stuff and apparently they started before xmas break cuz luna and ginny never came back.|||They never said, but they learned of it at the end of the year, on May 2. I would think beginning, however, since Death Eaters took over not long after school let out for the summer.