At my school, most professors will let you "drop a note." As the name implies, it is a system whereby an unprepared student can avoid getting called on for a day by leaving a note on the front podium before class ("Dear Prof. X, I am not prepared for class discussion today. Sincerely, Student Z"). Most profs would prefer knowing this in advance rather than waste time calling on you only to find out that they must seek answers elsewhere. But profs will also keep track of how many notes you drop and adjust grades accordingly, at the margins, so you don't want to do it too often.