declare @var1 int

set @var1=3

select schid, stuid from stu where stuid in (select stuid from (select * from stu where schid=@var1) a )

where (select * from stu) is your resultset query

SELECT studentID

FROM step1 [ step1 identifies all student with open attendances ]

GROUP BY studentID

HAVING (COUNT(*) > 1)