The aim of the Big Challenge is to create strong motivation among the students in your schools around learning English.
There is little point in registering the best students just to show everyone that they are the best. The real point is to register students, whatever their level, in order to create that "trigger" that sometimes pushes some of them to make a little more effort than usual to achieve a good ranking and, in the process, they discover that English isn't so difficult and that it really is within their grasp.