One possible version：
Dear sir,
I’m Li Hua, chairman of the student union of Sunlight Middle School. According to the agreement, we plan to send 30 students and teachers to have a study tour in London this summer. Now I’d like to tell you something about our group and something we’d like you to arrange for us.
Our group consists of two women teachers and 28 students, of whom 18 are boy students. It’s better for us to live in host families so that we can have opportunities to practise our oral English. As for the activities, it is suggested we have English class in the morning, and do some sightseeing or take part in some sports in the afternoon. In the evening we want to be free. On Sundays and Saturdays we’d like to visit some scenic spots within the country.
I wonder if you can send us your formal invitation as soon as possible so that we can go through all the procedures
Best wishes!
略