I have a SCHOOL table with the following rows:

ID CLASS CLASSID
-----------------------
100 BIO 000020
100 ART 000040

I need my info to display like:
BIO, ART
but the code I have displays two rows like:
BIO, ART
ART,BIO

How can I get rid of one of the rows?

Here is the code I created:

select a.CLASS + ',' + b.CLASS as SCHEDULE
from SCHOOL a, SCHOOL b
where a.id = b.id
and a.CLASS != b.CLASS