For users familiar with previous iterations of Duolingo's assignment features, you may recall that students had to complete all skills in order, up to and including the assignment you set. Now when assigning a skill out of order, that skill becomes unlocked for students and they can do it right away.
If you wish to assign skills to your students you may select the assignment option titled, "Level up a skill!" When selecting this option, students must complete a level within the assigned skill. A dropdown assignment list will appear with various skills to choose from. Select the skill you wish to assign and then select a start date and due date.
If for example, a student has already finished two levels within the assigned skill, that will mean they need to complete the third level for the assignment to count as completed. This will include more sessions and more difficult challenge types. If the skill being assigned is locked or hasn't been started by the student yet, it will become unlocked and the student will just need to finish the first level.