Its cool that your class goes over graffiti, but I doubt they are really knowledgeable as to the origination, the different tiers of work, separation of gang graffiti and pure writers, the numerous styles of work basing off location, separation and correlation of street artists and writers, etc.
Do some side work and show your teacher something new. Not hating, its just it amazes me how such a worldwide movement, although frowned upon, is still not acknowledged as important to people.
/rant