I know some informal words for disappointing people/doing something unpleasant to them, but I want to make sure that these sentences sound natural:
"I don't believe our teacher is gonna give us an exam today! He wouldn't let us down!"
"Our teacher wouldn't give us an exam today! That'd be a bummer!"
"Did John (a friend we trusted) really do that to you? What a bummer/letdown!"
If they sound awkward, what other words could I use in these situations?