Where's my Present?

Yesterday, Z's teacher told me that Z praised her, he said she is pretty. She said she was very happy and she will get a present for him (she said it in his presence). Later, I found out from him that he and 2 other classmates said that to her too.

Today when I picked Z up

Z : Mommy, Ms B did not buy me any present. I thought she said she is going give me one.
I have to think fast how to answer him
Me : It 's ok not get a present. When you compliment someone, you don't expect anything in return ok?
Z : But she said that.
Me : I know... Maybe she forget? Never mind, baby... If every kid says that to her and she has to buy for everyone, then Ms B will have no more money right?
At this point, I tried to divert to conversation to something else. Otherwise, what else can I say?

I don't understand why the teacher said that. She should know kids remember what they have said. I just hope he will forget about it tomorrow.