I like when students call me /prof/, like in /{senta, senti} prof, non mi e` chiaro .... ? / . There is no standard abbreviation for {professore, professoressa} unless on a mail address; ask the teacher!

thank you for your advice. I will take it.

you can say "prof." but maybe it's too informal.. depends what is your relationship with the teacher.. it's better professoressa :) I know, it's so long, but it's better and respectful