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.
So if you’re interested in education or just looking for some data to mess around with some for viz practice, NCES is a good place to start. I randomly came across some high school dropout data:
Does anyone else find it interesting that the dropout rate for those of mixed descent is in the middle? Although, I guess a few margin of error percentage points could easily change that…