I think it means everything good the other kids do to August that is nice (talk, getting to know him, and being his friend etc.) he will leave a good memory of them.

It may mean that, all the good things August has done, like going to a school to be like the rest of the traditional kids that go to school. He stands up to the kids that have disorders because kids are old enough to know that everyone is special and I think August needs to remind that their special in their own way.

It could contribute to the story by what Auggie is going to do, it’s going to leave a memory behind him.