Perhaps if you first prepared a relatively simple project for the students by having them use a specific part of ROS to do what you wanted.
For example, the suggestion of having the robot roll in a simple square. Though I might suggest doing line or maze following. Both of these seem more fun for students than rolling in a square which seems more like a homework problem.
Yes, I agree that all of the above are homework problems, but people react better when there is some fun and just a little competition involved, IMHO.
If you teach the general concepts of ROS and then concentrate on the parts of ROS that the students would need in order to achieve the goal, ROS might work out.
After I do the basics to learn ROS with the Waffle as ordered, I will attempt to replace the Joule with a NUC and give the Joule to my preordered Burger. After this, I will keep those machines for demos and build something more complex and fun.