Hold on, Ai-chan is pretty sure you didn't mean what you said here. Because what you said here is that 'many students were leaving THROUGH the window that he's looking at'. Ai-chan think what you meant to say "This young man sat at his desk, looking out the window. His gaze fixed on the front gate/main courtyard, through which many students were leaving for the day."
As for Ai-chan's feedback, your biggest problems are your grammar and your paragraphing.
This is too many words in a paragraph.
You could separate it into (together with correction):
First problem: Paragraphing. That's way too many things happening in one paragraph.
Second problem: Punctuation. Full stops where full stops are due. Dialogues with question marks should be enquiry dialogues, not statement dialogues. "I didn't expect you to bring home guests" is a statement, as if you have set a rule, but she broke it. If you want to sound bewildered, you would say, "I wasn't informed there would be guests, Luna?" as if you are accusing her of not giving you information.
Third problem: Capitalization and sentence structure.
Fourth problem: That's a very long time spent on cleaning duty. Ai-chan went to school in Japan. If we spent over 3 hours cleaning the classroom at the end of the day, you'd have to wonder what the other students on the days prior were doing. All we'd do at the end of the class were wiping clean the blackboard, dusting the duster, dusting the top of the cupboards, sweeping the floor and arranging the tables in order. Doesn't take more than half an hour even if you're doing it alone. Also, we usually have cram school every evening, between 4/5pm to 9/10pm, so if you spend too long cleaning the classroom, the teachers will come and berate you for taking too long before telling you to go home. Pretty much the only students at school after 4pm would be those with club activities.