Math Grades 9+
The Stock Market Game - This resource allows students to learn about stocks, bonds, mutual funds, diversification, and more while playing a simulation that mimics the Stock Market exactly! They get to compete with students across your district, region, and the entire US to see who can invest $100,000 the best. There are fall and spring sessions for competition, so sign up ahead of time, teach students a few lessons in advance so they know the basics, then start competing!
Alcumus Art of Problem Solving - Alcumus offers students a customized learning experience, adjusting to student performance to deliver appropriate problems and lessons. Alcumus is specifically designed to provide high-performing students with a challenging curriculum appropriate to their abilities.
Mathalicious - Teaching math through real world problems. Absolutely amazing site with 146 lessons for grades 6+, but costs $320 per teacher per year. However, they do have some free units that I've linked below that are good for high school.