I think you should use a tone that sounds very interested and very keen to know. Also, it should sound very 'socialised' and feisty voice.
Hope this helps :)
It would be too obvious for me to write the entire letter for you, but I'll include a list of points that you could include in your own writing style:
* Thanking Lee for his letter, and saying that you are happy that he is having a good time.
* Telling him roughly what you're studying at school (without being boring).
* Write about what you would like to do when you leave school.
* Finally, say that you miss him a lot and look forward to hearing from him soon.
Answer:
reading books gives me wing (I think this is the correct)
