You are not out of line to make the request because it's reasonable for a gifted child. It's whether you can prove it or not.
I second this - having documentation to show what our ds was capable of was what got us acceleration in math. We used Aleks - it's really handy for this because you can generate reports tied to each state's core standards. We found that type of documentation much easier for the school to accept than IQ scores and the typical achievement tests. If you think your child is ready now, you could sign up for a free trial of Aleks, and have them take the Algebra assessment test that kicks in at the start of the pre-Algebra course - that might give you all the documentation you need, *or* might let you know if there are areas your child needs to fill in before taking Algebra.
FWIW our school district also has an "algebra readiness test" which I think is a common test used across the US - but I can't remember the name of it this morning! Anyway, that's something you might ask about.
polarbear