I seem to find it easier to challenge capable students in math or solving puzzles than students who are passionate about writing.
It seems there are many online resources for math/puzzle solvers and not so many for writers.
Often the contests for writers include a fee or a requirement to purchase a bound book which can be out of a student’s price range.
Jerry Jenkins posted The Ultimate Guide to Writing Contests 2019. There are three dozen contests in his list. Here’s the one I found interesting.