Noetic Learning https://www.noetic-learning.com/mathcontest/

Math League http://www.mathleague.com

We usually had to do out of grade math contest. In 4th and 5th grade you could compete as a middle school student. Through middle school you can do High School competitions. Some might not let you compete but some do.

You have to check around. Talk to some of the math teachers in your area. Maybe a local Mathnasium might know of some local contest or possible host one.