Can high school grads work at an eikaiwa?

I'm not asking for myself, by the way!

I work at an international school in Japan and frequently deal with fresh grads and alumni.

Some of them wonder if they can do part-time work at an eikaiwa to teach English while they are in university.

The former students in question don't need visas as they are Japanese. They are completely bilingual and have native-level English.

Anyone have any insight about something like this?

by almostinfinity