Thank you very much for your corrections.

Here are two sentences for you to check.

1) Thank you for your prompt reply. How could I start and finish off my letter addressed to the principal of a high school?
2) Could you suggest to me an internet site where I could find schools interested in a partnershaft with other European schools?
3) Please remain seated and silent until the bell rings.
Keep the same seat next time. (Don't change your sitting order).
4) All of you should be able to pass the PET test at the end of their second year of high school and the FCE test at the end of their third year.

1. OK

2. Please suggest an Internet website where I can find schools interested in a partnership with other European schools.

3. OK

4. What pronoun do you need near the end of the sentence instead of "their"?

Here are the corrected sentences:

1) Thank you for your prompt reply. How should I begin and conclude my letter addressed to the principal of a high school?

2) Could you recommend an internet site where I can find schools interested in partnering with other European schools?

3) Please remain seated and silent until the bell rings. Keep your current seat for the next time. (Do not change your seating order).

4) All of you should be able to pass the PET test at the end of your second year of high school and the FCE test at the end of your third year.