Unfortunately, we don’t have a public API, but we’d advise tracking this feature request in our roadmap. You can vote and add comments on this feature request here: Public API for accessing, downloading, and uploading grades and submissions programmatically.
To make a comment on an existing feature card, you'll need to select the priority first (nice-to-have, important, or critical) and then a text-box will appear where you can leave some further feedback.
You'll also need to enter your email address, in order to ensure you get notified when your feature request is released.