
One possible version:
Dear Tom,
I'm sorry to know that you are in low spirits because you failed to join in the drama festival due to your weight.
To be honest,it is natural that we feel upset when this kind of thing happens.However,compared with our looksand weight,health is more important.So it is all right as long as we are fit.What's more,it's also possible for you tolose weight by working out and forming healthy eating habits.If necessary,you can look for someone to accompany you totake exercise after school.Whatever happens,we should keep positive.Only in this way can we have a happy life.
I hope you can cheer up soon.
Yours,
Li Hua