At this time, there is not a setting that allows a teacher to manually reset a student’s account. However, if a student decides that they would like to restart a language tree from the beginning, they may reset or remove the language from their profile.
For instructions on resetting or removing a language from a profile, please visit Duolingo Help Center: How do I delete a language from my profile?
Remember: ONLY reset if you are absolutely sure that you would like to restart the language tree. Duolingo is unable to restore course progress in the event a course is accidentally reset.