Your comments

It is a place that not many wizards do actually visit in their lifetime. Moreover once you're there, you do not even have many occasions to interact with other characters/players, so I do not believe it'd be a useful addition to the site. 

You can ask through an owl to the Headmaster or Minister of Magic of your site to delete your account ;)

You can already gain xp by roleplaying, sending owls and even talking in chats, and level-ups are displayed on your wall, so you do already have a sort of reward for being active. 

Moreover in the HP series, Housepoints are for students only, and mainly related to scholar achievements, so it wouldn't make sense to me if graduated adult characters could gain some and be awarded the House Cup - they're not even in Hogwarts IG anymore. 

I love the idea of the puzzle or minigame ! However I'm not really into the grading stuff - this could encourage people into speedgrading and not taking much care of the homework they have to read. 

I agree, something like a box to tick before updating to have a notif would be perfect. It would be a pain to get notified everytime we just correct a spelling mistake. 

You must immediately owl a staffmember of your site (the first half of the jobs list). They will take appropriate action. 

We can already get xp for being social (owls, topics and chat). Also I believe it wouldn't fit the series' idea - housepoints aren't given because you talk to your classmates, but for school results...

You can describe how the room looks like to your character in your topic, and according to the series it can be almost whatever you'd like it to be - just remember to check Gamp's Laws on the wikia and of course the realism rules, and feel free to ask any of your prefects if in doubt.