A very clever idea! A proofreader would go with the small errors. However, if the "author" of this letter is a student, most of the mistakes can be overlooked!
A sequel would be nice.

I would love to see the British Wizarding public's reaction to this letter. (No need to repeat the entire letter, which was very well written.)

You should definitely add a second chapter...

Great letter. It would certainly prompt a few parents to take action.

Additional comments might be that attempted murders took place multiple times where the DMLE was not called in to investigate, as it was a school matter. A troll let loose in the school, the boy-who-lived and a cursed broom, petrified students, an escaped mass murderer infiltrating the school, the boy-who-lived forced into a magical binding contract that was supposed to be for adult students only, etc. Why wasn't the DMLE called in for each and every one of these situations?

Please continue with this, I would love to read the reactions.

hhmm i think you should have added the bit about how the students are lazy and that snape had to use force to get them to do what he wants.

also we dont know if snape teached the basics in his classes.

snape could be a good teacher. but with older more serious people.

oh please put in a part on the reactions to this letter! that should be brilliant...

I would love to see a second chapter with the reactions of the people reading it. Could you have Sirius be proven innocent to the world, both muggle and wizard? Because I think that Sirius would not know what Harry has been up to because Dumbledore probably wanted Sirius in the dark. Because in the books Sirius did not suggest that Harry get better grades, to prank people who deserved it or try to get a date.

The entire Weasely family besides Ron who probably did not know about his adventures at school should know. Percy should be involved because he know works at the ministry and I have very rarely seen him being a better brother, though that is actually one story where he is a more protective big brother for Ron. And Ron has not been given sound advice or what to do for his education just him closing his eyes with Harry and picking which subjects his (quill?) landed on.

The Malfoy family, Draco's parents, probably should pay attention more to their son's education if they thought long enough on the subject.

Yes! Do that! And I agree with most of this article/story.

Yes, I'd be interested in reading.