Second graders can take the SCAT, mentioned by a PP. My kids took it in second grade for the Johns Hopkins talent search, but their scores also proved useful in advocating for math acceleration at school. This was in part because in addition to having the comparison with students two grade levels above, our school had SCAT scores for a decent number of other elementary students (they are automatically sent to the child's school unless you opt out), so the school could compare against those, too. It's true that the SCAT is a very specific type of test but the fact that it provides an above-grade-level comparison can be very helpful in advocacy, especially if your school won't administer an above-grade-level test.
