To which point is a paper considered as a "student contribution"?
In my experience, such declarations are typically defined in terms of authorship.  Thus, you will see, for example, a prize to be awarded for papers with a student first author, or for papers with only students as authors.

I would therefore generally interpret "solely written by" as meaning "all co-authors are."

If you want to be certain, however, it should not be any problem to request clarification from the organizers.