Ask HN: Good sources of math exercises for ~10-17 y/o?

I have two kids, 10 & 12, and I'd like to provide them with some math homework that is at least somewhat tracking their current and upcoming "Common Core" work. The school isn't assigning any homework, and I am disappointed at how thin their knowledge of the topics is despite getting good grades. The market, both online and in print, is awash with material, but much of it is really poor quality. Often it is trying way too hard to put some new/cute/fun spin on topics that really just need pen, paper, and practice. So many videos, contrived stories, artwork, ... my kid is 12, not 5.

Looking for any sites or book series folks have found useful. Ideally, these would be printable since they have to show their work on tests and are quite weak at this, especially with so much of their work exclusively on Chromebooks.