For a number of years we have had high schoolers involved in helping with grade school classes. Some of this is simply helping with checking, correcting, and doing other odd jobs. But there is also the option of teaching a class, such as a math class. Twice in the last three years this kind of arrangement has resulted in the “senior – turned graduate” then being hired part time the following year.
We also intentionally work toward having older elementary students helping out in younger grades as much as possible. This is usually doing small projects for the teacher, checking fact sheets, helping with corrections, etc. I think this kind of interaction between students of differing ages is a worthwhile endeavor.