You may have misunderstood the comment above. Generally people do not mix the multiple FROM tables with JOIN syntax.
I think you should write it this way instead. It should be easier to follow this way.
SELECT s.session_name semester
FROM lsx.elsp_session s
INNER JOIN lsx.elsp_course_offering co
ON co.session_id = s.session_id
INNER JOIN its.account ac
ON ca.student_number = ac.id
LEFT OUTER JOIN lsx.elsp_class_attendance ca
ON ca.course_id = co.id
and then see if you get syntax errors.