Right now, the way Duolingo is structured, it is not built for unlocking the skills in random order. The reason for that is that each skill is dependent on the student knowing everything that was taught up to that point. We use vocabulary from other previous skills in order to teach new terms and words. And we highlight new words as they are introduced to ease the student into the concept.
For example, if a student is ready to tackle skill 20, the system assumes he/she is familiar with everything taught in skills 1-19 and is arriving at 20 with appropriate scaffolding. If not, the lessons might prove difficult and the frustration could affect efficacy.
However, if a student is completely familiar with the content of skills 1-19 but just doesn't want to go through each skill, they could take shortcuts and unlock up to that skill in a few minutes, by taking testing out at a checkpoint or taking a skill test. That's a way you could get them into the appropriate place if they didn't take a placement test when they first started.
Please note that this does not mean we won't offer this ability—it just means that, considering the way things currently are, it's tricky to do this. We've heard of many teachers doing either the shortcuts (checkpoints), adapting their curriculum a little to make it a closer match to the skill tree, and even some that are sending Duolingo assignments completely independently of their curriculum because it did not initially match (or because students are each reinforcing their language skills at their own level). They claim student performance improves either way.