When things like this happen all the time.

Me: *sees teacher* Okay, okay. He's probably going to say good morning to me. All I have to do is respond back. Two words. It shouldn't be hard.
Teacher: Good morning!
Me: *opens mouth to reply*
Me: *can't make a single noise*
Me: *walks by without saying a thing*
Me: *realizes I probably seem really rude and stuck up*
Me: Fantastic.