Hoagie's talks a little about grade equivalents (http://www.hoagiesgifted.org/why_test.htm), and one of the points they make is that an "average" high school senior has math skills equivalent to Algebra I.

There's a recent article here that might be interesting, but it didn't interest me $25 worth: http://jpa.sagepub.com/content/15/3/214.abstract