Among the states with a public school count of 20 and below, list their race.
public refers to control = 'Public'; school_count < 20;
SELECT DISTINCT T2.race FROM state_sector_details AS T1 INNER JOIN state_sector_grads AS T2 ON T2.stateid = T1.stateid WHERE T1.schools_count <= 20 AND T1.control = 'Public'