From 1978 to 2008, the US average mathematics score on the National Assessment of Educational Progress (NAEP) continuously improved.It is difficult ascertain the cause of this improvement due to the multiplicity of factors changing during this time period.One can hope that improvements are due, in part, to implementation of reform-based instruction in response to reform movements that occurred over this time period.
Source: http://nces.ed.gov/nationsreportcard/lttdata/dataset.aspx (select national, all years, all students)