https://www.arml.com/ The American Regions Math League (ARML) is a nationwide high school mathematics team competition held simultaneously at four locations across the United States.

https://artofproblemsolving.com/community/c5h3086665_25th_elmo_2023_signups The ELMO is an olympiad test similar to both the USAMO and IMO in format. It is written, administered, and graded by returning MOPers for those attending MOP for the first time. However, because there are only finitely many people who can attend MOP each year, for many years the competition has also been posted and run on AoPS.

https://bmt.berkeley.edu/ Our live fall tournament offers teams of high school students the opportunity to share their interest in mathematics with like-minded individuals and to get a taste of the concepts studied in higher mathematics.