For elementary students:
1. MOEMS (Math Olympiads for Elementary and Middle School) are great (http://www.moems.org/ ). The contests run Nov-March. Their books are fantastic.
2. Continental Math League are also great (http://www.cmleague.com/product-category/cml/ ). A few CCSD schools do them. They also run Nov-March.
3. Noetic Learning Math Contest (http://www.noetic-learning.com/mathcontest/ ). Again, a few schools in Cherry Creek do these. The contests are one in November, one in April.
4. Colorado Math League (http://www.mathleague.com/). It’s all about Number Sense. Fantastic contest! And books are also great. Contest is in April.
5. Math Kangaroo is another great one, in March. Kids can take it at various “centers” around Colorado. http://mathkangaroo.org/MathKangaroo/Results.xhtml
6. Beast Academy. Great curriculum for advanced math (and contest math). (https://www.beastacademy.com/ )
7. Of course there's Prodigy, Blooket, Reflex Math, Quizizz and MANY other fun online math resources....
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For middle school students (or elementary students who want an EXTRA challenge):
8. Many of the above contests and resources also have middle school editions.
9. Art of Problem Solving (or AoPS).( https://www.artofproblemsolving.com/ ) This is THE first place to go. It has a wealth of resources, and sometimes it takes time to find them all. The MathCounts Trainer is great. So is “For the Win.” All the old AMC contests (and many others) are available, with solutions (both official and ones uploaded by users/students).
10. The AMC8 is in November. It’s the beginning of the AMC ladder of contests, which ends with the IMO (International Math Olympiad) where each country sends its best six students. All the old AMC8 contests are available (with solutions) here: (https://www.artofproblemsolving.com/wiki/index.php?title=AMC_8_Problems_and_Solutions )
11. MathCounts is really the top middle school math contest program. Each school brings 10 students to their “chapter” competition (in February), where individuals and teams compete. It’s SO great! Old school handbooks are all over the internet–the most recent is here: https://www.mathcounts.org/resources/school-handbook and others are very “Google-able”
12. Colorado Math League. Same as #4 above, but the middle school contests are in February. Again, their books are fantastic. http://www.mathleague.com/
13. The Purple Comet is a great team contest in April. (http://purplecomet.org/ )
14. The Cherry Creek Math League starts in September, and all Cherry Creek Middle Schools participate. Each middle school has contests and practices every Monday, and each school should have more details.
15. The Colorado Math Circle in Boulder (http://www.coloradomath.org/ ) has monthly meetings which are fantastic.
16. The Denver Math Club (started and run by CCSD students) also has monthly meetings. Get on the mailing lists for these last two clubs to get their updates http://denvermathclub.weebly.com/
17. The AoPS book curriculum!! If I have one recommendation for middle school advanced math students, it is to BUY some AoPS books and work through them. They are all great. Full of great problems. Art of Problem Solving, Volume 1 is a good place to start: https://www.artofproblemsolving.com/store/item/aops-vol1