The order of Duolingo's trees are scientifically tested and known to be effective. We are constantly running tests to find improvements and update the trees accordingly as soon as we make new discoveries. We also adapt to each student's needs—every student takes a placement test and moves forward at their own pace, with exercises tailored to strengthen the skills that need the most work. We present them with the right challenge level to help them learn without getting bored or discouraged.
Duolingo for Schools gets updated often and has changed a lot over the past few months... this is going to keep happening (we have a variety of tweaks in the works to make it much better… and can’t wait to share them with you). We listen to teacher feedback in person and in the forums in order to inform our next decisions, and recommend you come back often and meanwhile keep an eye on the Educator’s forum, which is where we announce new, shiny highlights. :)