Originally Posted by ohmathmom
You can participate unofficially in the Math League as a homeschooler.
Originally Posted by ultramarina
The one we are familiar with is http://www.mathleague.com/.

(DD considered doing this but decided against it. I haven't looked at the problems.)

Okay I haven't heard of this one. Any reason to "decide against it".

There actually seem to be a lot of mathematics competitions out there. Generally, how does one choose which ones are most worthwhile (for a very strong math student)?