There's one key if you're joining the 2 WHERE clauses of your SELECTS and that's parentheses.

SELECT...
WHERE (1st join condition) OR (2nd join condition)

Without the parens you'll get, .... who knows but it won't be what you want.

RE: # of students returned. By default, the UNION eliminates duplicate records. In my cursory review of the code, it seems as though the 2 WHERE clauses are mutually exclusive. But that may not be the case, and if you have a the same student-class-etc. returned in both SELECTs, you'll get only one record for both instances in the UNION.