Sorry couldn't write the whole thing, hope this could help.
1st- you greet her in the email
2nd- you tell her the purpose of the canteen day held in your school
3rd- what time did all the prefects reach the school to decorate the canteen
4th- the food sold
5th- how many people were present, the performances held
6th- other activities like games
7th- give a reason to leave and finish it there
Try to keep it short and simple. Trust me, that will provide more marks.