Seems simple enough. If it bothers you, ask why the artist wasn't credited with the work. It does sound like he did it
for you guys though. I give you credit for caring. Just ask your teacher. You'll feel better for knowing.
BTW, most murals are quite the opposite. The designer gets all the credit while it's his/her underlings who do all the work and get none.
Podium
Well a while back my advanced art class was asked to paint a mural for our school. It was great but i felt like we werent actually doing much. All the credit was given to us and not the artist who Actually designed the mural. He did most of the work while we painted one little area as a class. The mural was amazing but i feel like we didnt give the artist nearly enough credit. Is that unfair? Its been bothering me and i would like some other peoples opinions.