"курсы полького" is in genetive case, right? Is this because of "ходить на" always requiring genetive?
The word курсы is in accusative case (в винительном падеже), because it serves as an object here. The preposition might confuse you a bit, but you can easily replace ходить на with посещать (visit). Having done that it is more clear that the курсы is object wich the action is acted upon: Я посещаю (whom/what? - кого/что?) курсы. But the word польского is in genitive. If it was adjective for курсы like Polish courses (польские курсы) then it would accept the form of the noun. In our case the full phrase is "Я хожу на курсы польского языка"- "I go to courses of Polish language". As the "language" is omitted, the word польского takes the role of a noun and describes the object курсы. And the of-connection makes it genitive. I tried to describe it in detailes, hope I did not confuse you here