While Spanish has a large number of heritage speakers, it is also the most commonly offered foreign language in US high schools. The SAT subject foreign language tests that are most heavily loaded with heritage speakers are Chinese and Korean, since those languages are relatively rarely taught in US high schools.
Percentiles of SAT subject tests: https://secure-media.collegeboard.org/sat/pdf/sat-subject-tests-percentile-ranks.pdf
Note also that 800 on math level 2 is only 79th percentile, but that is due to a self-selected group of test takers (i.e. those good in math and on the +1 or higher math track).