The school strata were assigned separately for public and private schools (as is generally done in NAEP studies).
The stratification of the school frame was an implicit stratification, using systematic sampling through a sorted file. Sample design staff used a serpentine sort order. The hierarchy of sort variables was as follows:
public or private schools,
certainty PSU or non-certainty PSU,
for certainty PSU schools, NAEP region, and for noncertainty PSU schools, PSU stratum,
for public schools, type of location, and for private schools, school type (Catholic, Lutheran, nonreligious private, other religious private, conservative Christian), and
race/ethnicity strata with grade enrollment as the final sort variable, or alternatively race/ethnicity percentage by itself, as the final sort variable.
Race/ethnicity percentages for each school are the summation of the school percentages of Black, Hispanic, and Native American students. The details of this process are given in the pages linked to this web page, for public and private schools separately.