You were doing Ok at putting together a somewhat believable semi original story up until these last two chapters...
That DADA lesson was over the top... aren't they suppose to be 1ST years??? they wouldn't have known anything about Forgivables or Unforgiveables at this point without exposure to them. To threaten them??? WAY over the TOP..
To many original characters... with changing Harry's name and personality you've essentially rewritten him as an original character and same with the introduction of his partner in crime this Valkarie..
BUILD the story and on the characters you have... put a plot together give us scenes, dialog and background...
The lesson was good in that you had interaction between teachers and students... just the spells were too high of a level for where you started the story.
Keep Writing...