i'm not gonna write everything for you cause im lazy but i will give you some help and somewhat of an outline - you don't have to follow it completely, just some ideas
if you're still stuck after reading this or don't want to do it, just ask and i will write it for you if you want me to :)
introduction:
introduce yourself
why are you writing this letter?
what will you discuss in your letter?
make sure to be friendly and respectful in the email by not pushing the celebration on the principal
paragraphs:
points on why your class should celebrate family week or friendship week in your school
points for family week:
- to recognise the importance of family
- closest friends
- will always be by your side
suggestions for points on friendship week:
- to strengthen relationships
- recognise the importance of friends
- friends are like family
conclusion:
overall, i think that........
it would benefit....
try and relate the conclusion to yourself and think why the celebration is important to you and why might it be important to others