What are the first names of the students in that classroom that begin with the letter "S"?
SELECT FirstName FROM list where firstName like " S%" and classroom in  ( SELECT classroom FROM list GROUP BY classroom ORDER BY count ( * )  DESC LIMIT 1 ) 
first names of students = SELECT FirstName; that begin with the letter 'S' refers to WHERE FirstName LIKE 'S%'; in that classroom refers to filtering by a specific classroom; the classroom with the most students is identified by ORDER BY COUNT(*) DESC LIMIT 1 within a subquery.