Duolingo for Schools does not grade a student’s work. As a student successfully progresses through the tree, they unlock the next lessons and units. A student won't advance unless they have mastered the previous skills. Essentially, each assignment is considered a complete (mastered the skill) or incomplete (ended the lesson before finishing the skill) and no letter or numeric grade is recorded.
Duolingo does provide data such as points earned, assignments completed, course tree overview, and streak days for you to assess student progress.