If you allow your high school students a way to offset that one test they bombed out on, consider making extra credit a proactive, rather than reactive, choice for them. Post at the beginning of the course, or hand out along with the syllabus, a list of projects or activities they can complete to boost their overall grade. But assign point value on a sliding scale: a project worth 10 extra points the second week of the semester might be worth 6 points halfway through, and only 4 points when there are two weeks left to go. It might force them to do extra work before they need it, or realize the value in not procrastinating.