OOC!

There is no evidence at all that Percy Weasley has any kind of mental instability - apart from being a choleric person (seems to be a character trait of the whole family).

Assuming Ron's tale is correct, he was excited about the promotion, instead of receiving congratulations, Mr Weasley basically accused him of being a spy (implying he didn't deserve the promotion), both of them started arguing, the fight escalated into a shouting match and Percy left immediately, i.e. before he or his father had a chance to calm down.

And after that he was too stubborn to come back home.

Things like that happen every day in all kinds of families.

This is wonderful. I love it, and it's actually very, very in character. It describes the disorder perfectly. It comes to an easy close and begins perfectly, and has just the right amount of substance. Great work, overall. Now, for the marking process.

Spelling & Grammar - 10/10. Wonderful mechanics, very easy to read.

Characterisation - 10/10. You captured Percy perfectly in my eyes.

Use of Topic - 8/8. The disorder was captured carefully and thoroughly, while still maintaining the feel & tone.

Flow - 5/5. Very easy to read, made sense from beginning to end.

Believability - 3/3. It was sensible and in character.

That gives you a total of 36/36, full points. Amazing work, & definitely going in my favorites!