Russian School of Math has the RSM Olympiad, if you live in a city that has a RSM center. Their questions are pretty challenging. I think anyone can attend the competition (I think they posted their questions on their website). They are also very good for math enrichment.

http://www.russianschool.com/olympiad

It is also very advanced math; sometimes I think it is a toss-up whether RSM math olympiad is harder than Math Kangaroo.

Last edited by mom2one; 06/04/15 11:05 AM.