The short letter task on the gaokao is a formality exercise. The scorer wants to see that you can open, deliver information, and close in appropriate register. Ambition costs more than it earns here.
Paragraph one: state your purpose in one sentence. 'I am writing to invite you to…', 'I am writing to apply for…'. Do not warm up. Do not open with a greeting question ('How are you?').
Paragraph two: give the specifics — dates, times, expectations, requests. Two or three sentences. Use linking words: 'firstly', 'in addition', 'therefore'.
Paragraph three: close politely. 'I would be grateful if you could confirm your attendance.' 'Looking forward to hearing from you.' Then 'Yours sincerely / Yours faithfully' — sign off with the correct one.
Memorize a bank of ten polite formulas. Rotate them across practice essays so no two of yours start the same way. Graders notice.