Student or Learner
I´ve heard Americans saying yep instead of yes... Is it common/avoidable? What about yah (with an open a)?
yah, with an open a, as in German.
(Not a teacher)
Yep, the word "yep" is widely used in the U.S. as an informal way of saying "yes;" the equivalent word for "no" is "nope." It probably isn't used much in very formal conversations although I doubt it would be noticed. I don't know how much yep is used in other English-speaking areas but it probably has some use.
"Yah" (or what was perceived as that sound) was initially used by Scandinavian newcomers in the U.S. whose word for "yes" was "ja." It doesn't appear to have any widespread use today.