What should I say?

My friend is going to ask his Japanese college professor whether I(who is not enrolled in the unit) could sit in their classes and get some more Japanese exposure.

If the professor is kind enough to let me join, what should I say in Japanese to express my gratitude, as well as apologizing for the hassle I'm causing?? Are there any customs that I should follow (perhaps a thank you gift at the end of semester or something)?

Thanks in advance!! :D

Just hand in a thank-you-letter with your improved Japanese.

You will be a non-credit student or occasional student called "Cho-ko-sei". Cho-ko-sei is generally accepted by lecturer in university classes.
At end of class, you say "Cho-ko sasete itadaki arigatou gozaimashita"(Thanks for accepting me as a cho-ko-sei).

You get on the floor and beg for forgiveness than ask for permission