Things can be left unsaid if there's really no big deal in keeping it to yourself.
Let's say you failed an insignificant 5-point quiz. Considering that all the assignments and tests you'll take for a class will at least form 1000+ points in total, a single quiz that you failed will do almost nothing to hurt you.
If your parents ask about it and you KNOW they'll freak out to an extreme amount, even though they don't realize that it practically won't do anything to hurt your grade, it'd be better if you make up a viable excuse by saying "the teacher/professor hasn't graded it yet," or something along those lines. Save yourself the trouble of ruining your WHOLE day by being yelled at for such a stupid and insignificant little thing.