Raw, fine-grain data is still a bit hard to come by. Summary statistics (i.e. data that came from some analysis), on the other hand, are often easy to find. A lot of the time the data is already online or just a simple phone call away.
The National Center for Education Statistics, a part of the U.S. Department of Education, offers a bunch of data including, but not at all limited to, poverty and math achievement, average science scores overall and by grade level, and quantitative literacy.