Dear Dad,
I was so happy to get your letter. I am sorry I haven't answered earlier but I was really busy with my school.
You asked me to tell you about my school. Well, I finished the school year very well. I have to pass many different exams, since I'm in 9th grade. I plan to go to the 10th grade next year. Holidays I will spend with my friends and parents. I think I can come to visit you.
Unfortunately, I'd better go now as I've got a lot of homework to do. Keep in touch!