There is also MathCounts, usually run separately at each school (and a student can do both).
And Mathalon, a one-off team event in October in Laurel MD, organized by a teacher or a parent at each school or non-school club. It's pretty informal organizationally (and has no website

).
And other events like MathLeague.com and MOEMS and AMC8, which are administered at school during class or after school, but not a season of many practices and meets.