When moving up, we should be going positive by adding.
When moving down, we should be going negative by subtracting.
Sprite is moving up when we substract and moving down when we add. Students had to switch their blocks to the opposite logic to make it work. Right and left (.X) work just fine.
No. 
Actually you have to increase the Y pixel coordinate to move downward.
Why? The Sprite is on a Canvas, isn't it? How the Canvas is mapped explained.
The Canvas docs:
https://ai2.appinventor.mit.edu/reference/components/animation.html#Canvas
This is one of those historical precedent stories, like the one about the distance between railroad tracks traced all the way back to ancient Rome.
This coordinate system tracks how the first Cathode Ray Tube (CRT) displays moved their electron beams across and down the screen while drawing a screen image.