Can you please check my sentences together with a few more points I need to include in my project work?

1) I really hope you had a good journey back home.
All our teachers have been enthusiastic about your visit and your principal's words about the project!
2) They will never forget your presence here.
However, I still believe that we should have included Finland as a partner, too.
3) By the way, I'm attaching to my email the presentation of the Finnish school as received by the school principal.
4) Over the weekend I mananaged to write a few more things on the objectives of our project. I think you should organize them in: general objectives, scientific objectives and linguistic objectives.
5)cooperate with students from different class and age groups to complete their tasks
6)increase their self-esteem, autonomy and team spirit
consider the project as part of the
regular activities of the school curriculum

I really hope you had a good journey back home. All our teachers are enthusiastic about your visit and your principal's words about the project! They will never forget your presence here.

(I have deliberately omitted the negative sentence here.)

I'm attaching the Finnish school's presentation as received by the school principal.

4) Over the weekend I mananaged to write a few more details for our project's objectives. I think you should organize them into these categories: general objectives, scientific objectives, and linguistic objectives.

5) work and study cooperatively with students from different class and age groups to complete their tasks

6) increase students' autonomy and team spirit
(I removed "self-esteem" because I believe that all people need to develop self-esteem by means of success in their work/study. To work on self-esteem as a separate topic is to work on false or insincere praise -- not a good thing. " target="_blank">http://www.functionalpathtrainingblog.com/2010/06/lowering-the-bar-false-praise.html)

consider the project as part of the
regular activities of the school curriculum

Sure, I can check your sentences and provide suggestions for improvement.

1) I really hope you had a good journey back home. All our teachers have been enthusiastic about your visit and your principal's words about the project!

This sentence looks good, but you may want to consider adding a comma after "home" for better clarity: "I really hope you had a good journey back home, as all our teachers have been enthusiastic about your visit and your principal's words about the project!"

2) They will never forget your presence here. However, I still believe that we should have included Finland as a partner, too.

This sentence is fine. It effectively conveys your point about the importance of including Finland as a partner.

3) By the way, I'm attaching to my email the presentation of the Finnish school as received by the school principal.

This sentence is clear and concise. No improvement needed.

4) Over the weekend, I managed to write a few more things on the objectives of our project. I think you should organize them into general objectives, scientific objectives, and linguistic objectives.

This sentence is good, but you may want to add a comma after "weekend" and rephrase the last part for clarity: "Over the weekend, I managed to write a few more things on the objectives of our project. I suggest organizing them into three categories: general objectives, scientific objectives, and linguistic objectives."

5) Cooperate with students from different class and age groups to complete their tasks.

This sentence looks fine, but you may want to consider rephrasing it for better clarity and structure: "Students will collaborate with peers from different class and age groups to complete their tasks."

6) Increase their self-esteem, autonomy, and team spirit while considering the project as part of the regular activities of the school curriculum.

This sentence is clear and concise. No improvement needed.

Overall, your sentences are well-constructed and convey your points effectively. Good job!