Music must be in public domain

Some of the online challenges have a requirement for backing music to be in the public domain. Does this mean music created by the students for their entry must be released under a public use license too?

If the students are creating an original piece of music (not covering an existing tune or including samples from existing pieces, but have composed an entirely custom piece of music), they do not have to publish it under a public use license.
The rule for using public domain music is to prevent legal issues with copyright infringement. You may use an original composition of your own without penalty.