Comic-book-style advanced math for grades 1-5 from Art of Problem Solving, available in print and as an online platform. A good fit for microschools serving strong math students who need depth, not acceleration alone.
Mastery-based math program built around physical manipulative blocks and short video instruction, from early counting through calculus. Its skills-based (not grade-based) levels suit mixed-age classrooms.
The Dimensions Math series from Singapore Math Inc. uses the concrete-pictorial-abstract approach that made Singapore's program famous. Strong conceptual foundation for PreK-8, with teacher guides written for non-specialists.