I didn't read the previous posts. The idea is really similar, but I'm not using a temp table (I probably should). This would be a lot easier on 2005+

SELECT s.*

FROM #student s

JOIN (SELECT 1 priority,'IT' class UNION ALL SELECT

2,'Science' UNION ALL SELECT

3,'Arts' UNION ALL SELECT

4,'Commerce' UNION ALL SELECT

5,'Geography' UNION ALL SELECT

6,class

FROM #student

WHERE class NOT IN( 'IT','Science','Arts','Commerce','Geography')

GROUP BY class) p ON s.class = p.class

JOIN (select rollno, MIN( priority) minpriority

FROM #student s

JOIN (SELECT 1 priority,'IT' class UNION ALL SELECT

2,'Science' UNION ALL SELECT

3,'Arts' UNION ALL SELECT

4,'Commerce' UNION ALL SELECT

5,'Geography' UNION ALL SELECT

6,class

FROM #student WHERE class NOT IN( 'IT','Science','Arts','Commerce','Geography')) p ON s.class = p.class

GROUP BY rollno) o ON s.rollno = o.rollno AND p.priority = o.minpriority