I need to ask you a few more things about my letter. I hope you can have a look at them.

1) I'm terribly sorry that I didn't manage to write you (or to you?) earlier. ......
2) Some of our science teachers would like to include twenty students taken from one single second class, namely the current second B, instead of individual students (high achievers) selected from different second classes. (The whole class will be included - I need to express this)
3) During the two or more annual meetings, they will summarise the results achieved (by them)and exchanged information with their French and German partners.
4) We believe that our English teachers could refer to the website ... to teach science concepts in English.

1) I'm terribly sorry that I didn't manage to write you to you earlier. ......

2) Some of our science teachers would like to include twenty students taken from one single second class, namely the current second B, instead of individual students (high achievers) selected from different second classes. (The whole class will be included - I need to express this) <~~I think you've already expressed it fine -- if you need to add emphasis to make your point clear, then make the words "one single second class" in bold.

3) During the two or more annual meetings, they<~~Who are "they"? will summarise the results achieved and exchange information with their French and German partners.

4) OK

1) I apologize for not writing to you earlier. To determine whether to use "write you" or "write to you," you can consider the verb "write" and its preposition "to." In this case, you are communicating with someone, so it would be correct to say "write to you."

2) Some of our science teachers propose including twenty students from the same second class, specifically the current second B. To express that the entire class will be included, you can say "the whole class will be included" or "all students from second B will be included."

3) During the two or more annual meetings, the teachers will summarize the results they have achieved and exchange information with their French and German partners. To clarify that the results are achieved by the teachers, you can say "the results achieved by them."

4) We suggest that our English teachers refer to the website ... to teach science concepts in English. The phrase "refer to" implies directing someone's attention to a particular resource or information.

If you have any further questions or need additional assistance, please let me know.