As a teacher, I have two requests that would make Duolingo better for my students.
The first would be the ability to disable testing out of subjects. The problem that arises with this option is that my students will skip the lessons and use Google Translate to aid them in testing out. I want them to go through the lessons, but there is no way for me to assure that this happens.
The second would be harder to implement, and I'm sure it has been asked about before, but it would wonderful to be able to assign specific subjects without regard to student progress to coincide with my curriculum, or to even be able to construct our own custom trees. I realize this would be difficult to implement due to the way the materials build upon each other in Duolingo, but it would be very useful as a teacher.