Although I don't have any idea about what this query is supposed to
return, I think your query can be safely rewritten as:

SELECT
session_id,
campus_id,
SUM(student_count) AS student_count,
SUM(CASE WHEN session_id=student_starting_session_id
THEN 1 ELSE 0 END) AS new_student_count
FROM (
SELECT DISTINCT
session_id,
student_id,
campus_id = (
SELECT student_campus_id
FROM d_bi_student
WHERE a.student_skey=d_bi_student.student_skey) ,
student_count = day0_class_count,
student_starting_session_id = (
SELECT student_starting_session_id
FROM f_bi_student_statistics
WHERE a.student_id = f_bi_student_statistics.student_id
)
FROM f_bi_registration_tracking_summary a
WHERE day0_class_count > 0
) AS z
GROUP BY session_id, campus_id

There may be further improvements, but without understanding the
meaning of your tables, that's the best I can do.

Razvan