Could this be used as an actual school program?
and - yes - the short answer is yes.
However there are books and books worth of the long answer.
And it depends in which school you are answerable to, and according to things such as Curriculum structures and formats specific to the school and region and country you are answerable to.
Duolingo is a resource with a great deal of depth and breadth and potential for one of the most important elements in education - especially for language - serendipitous learning.
Aiming to be engaging and fun and relevant.