If Master Onion was just a dream, why is he at the concert?
He isn't there. He is in Lammy's imagination. "It's all in the mind."
Since Lammy warped through time, she never actually met any teachers besides Paul Chuck and Yoko Teriyaki. Why are the other teachers at the concert.
Explanation 1) Since Yoko Teriyaki liked Lammy's performance so much, she transported herself to concert and asked other people to come. Those people were the other teachers.
Explanation 2) She didnt warp through time, she just got launched to the island and went past the other teachers on the way. If she did warp through time she wouldve say her past self in the plane and putting out the fire.