In competitions, for all of Science Olympiad (includes binders etc.), or in their whole lives (includes schoolwork etc.)?
Number of words written for a test question varies between 0 (multiple choice & calculation questions) and whole paragraphs.
So let's say 1e5 tests in the year (following previous question) times maybe an average of 30 written questions per test, times a 20 word sentence per question would make the answer 8.