How many Rock Hill City School projects have teacher donors?
Rock Hill City School refers to school_city = 'Rock Hill'; teacher donors refers to is_teacher_acct = 't'
SELECT COUNT(DISTINCT T1.teacher_acctid) FROM projects AS T1 INNER JOIN donations AS T2 ON T1.projectid = T2.projectid WHERE T1.school_city = 'Rock Hill' AND is_teacher_acct = 't'