@Kelly Cray - As you have noted, according to the Board Co-owner Help Center article, this feature is restricted to Consultant and Enterprise Plans:

If you have followed the steps in the article to try adding your co-teacher to the board and do not see the option to make them a Co-owner, then there is no way to force this.

Alternative Options

If you are both not on the board at the same time, you could make your co-teacher the owner of the board - they would them have to transfer ownership back to you. More on this here:

https://help.miro.com/hc/en-us/articles/360017572194-Access-rights-and-board-ownership#Transferring_board_ownership_to_another_board_collaborator

Wish List Idea

You could request this feature be added to the Education Plan by adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know.

If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.

Lastly, don't forget to vote for your own Idea.

Thank you Robert.  I took your advice and added it to the wishlist!  If others would like this feature please upvote here 

https://community.miro.com/ideas/allow-edu-educator-plans-to-have-board-co-ownership-8679