I'm from the state of Texas in the USA, and you probably would get a very big smile from people if you said that there. In New York City or Los Angeles, however, people probably would think you were uneducated (just like you probably should not elide the sound of the letter "r" in Texas). It should not be offensive anywhere in the USA, but using it with a teacher may seem too informal or disrespectful, depending on the circumstances.