What is the highest number of students with a teacher:student ratio of 20.8?
CREATE TABLE "schools" (
  "school_name" text,
  "city" text,
  "students" real,
  "fte_teachers" text,
  "pupil_teacher_ratio" text,
  "school_level" text,
  "low_grade" text,
  "high_grade" text
);
SELECT MAX("students") FROM "schools" WHERE "pupil_teacher_ratio"='20.8';