The school has a kindergerten, a primary school, and a secondary school so, I could see off three different grade s. The school has a kindergerten, a primary school, and a secondary school so, I could see off three different grades.

It's more common to say "high school" than "secondary school"

I eat lunch with grade six every Saturday because there are only two people (including me,) in the same grade as me. I eat lunch with grade six every Saturday because there are only two people (including me) in the same grade as me.

I was really moved inby this graduation . I was really moved by this graduation.

sSome teachers who had graduateding students were criedying silently . Some teachers who had graduating students were crying silently.

thSome students were in grade nine , made a band and played a song . Some students in grade nine made a band and played a song.

As a back ground, the memories from when they were kindergeartener and by yesterdays were shown. As a background, the memories from when they were kindergarteners were shown.

I'm not 100% sure what you meant in this sentence, but I guessed you meant they projected photos/memories from when these students were in kindergarten on a screen?

According to their final speech, two of them were together from kindergearten. According to their final speech, two of them were together from kindergarten.

KindergArten, like gArden. It's German for a garden of kids

It was an amazing performance . It was an amazing performance.

No space needed before the full stop/period.

I was so happy because I could see off them . I was so happy because I could see off them.

On the other hand, I was a little sad because I cawon't sebe able to see the people who wasere in grade nine any more. On the other hand, I was a little sad because I won't be able to see the people who were in grade nine anymore.

After graduation, I had a thank you party and we ate sandwiches , potatoes, sushi , and so onmore. After graduation, I had a thank you party and we ate sandwiches, potatoes, sushi, and more.

Most of parents and students saied it was more richly mealsbetted catered than last year. Most of parents and students saied it was betted catered than last year.

Better catered = the catering (food provided by caterers) was better than last year

I was attend ined last year 's party , andso I couldan compare them and I also thought the same thing. I attended last year's party, so I can compare them and I also thought the same thing.

I could also borrowed three more books than usual from the library, because we wouldare going to be in the Spring holidays soon. I could also borrow three more books than usual from the library, because we are going to be in the Spring holidays soon.

I borrowed two taext books for next grade from the library . I borrowed two text books for next grade from the library.

I look foward to going to the Japanese school even more now. I look foward to going to the Japanese school even more now.