I dreamt that I was in the classroom and tried to get the teacher's attention by yelling Sir! My yelling woke me up. what does it mean?

Most dreams don't mean anything at all, and are just incoherent ramblings of the sleeping brain. Sometimes dreams are the subconscious trying to make the conscious mind notice something it's ignoring in waking life, and other dreams have psychological interpretations that are as obvious to the dreamer as to us. I don't believe there is any universal key to dream meaning, though in the Dream Interpretation forum there will of course be many others who disagree with that--I think meanings are personal to the dreamer. I don't normally say so, but when you have no other answers at all, or have only meaningless ones or the website spammer, I'll chime in then.

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It might mean that somebody is trying to take you to places you don't want to go and you need to look around you more to figure out what people's motives are.