I agree, Bwandungi.  & think that sometimes teachers toss out rules so students will find creative ways of breaking them. But I understand the impulse. Nothing is more boring than someone else's random dream--if you begin with a dream, you'd better make sure it really has a built in reader hook.  I think that's the point the teacher was making?

Oi! I dislike rules. Some of them are just asking to be broken!

What if the dream does a great job of introducing the character? What if the dream is used as the hook? What if the dream provides the setting or the theme of the book? What if it is a short dream?

What about epistolary novels? Would that same person say that the first letter would be sufficient in investing the readers in the characters writing the letters?

I think it always depends on the situation and try not to follow rules.