How would I do this?

Easiest method is to make a broadcast which makes each object wait until a variable is lower than the objects y-position and then it goes to the front. Then run a loop to set the variable to 180 and step it down to -180 - steps of 5 or 10 may be OK rather than stepping 1 at a time. Add a wait 0 seconds and repeat if necessary.

Depends a bit on how much things are changing though. If only one or two objects are changing their layer there will be better ways to do this.

Here maybe try this:

forever
if <(y position)  <  [0]> then
go back (10) layers
else
go to front
end
end

Put this in each sprite you are using.

when green flag clicked
if <happy> then 
have a good day
  

else
broadcast [become happy v]
end

We can make 2* global variables, and set them to the y position of the sprite. Then, we can use an if statement to check every single frame whether or not one sprite is higher than the other.
*two is for demonstration purposes. Make as many variables as there are sprites/clones.
Kind of like this:

// in sprite 1
when green flag clicked
forever
set [sprite 1 y pos v] to (y position)
end

// in sprite 2

when green flag clicked
forever
set [sprite 2 y pos v] to (y position)
end

// in sprite 1

when green flag clicked
forever
if <(sprite 1 y pos) > (sprite 2 y pos)> then 
go to front
end
end

// in sprite 2

when green flag clicked
forever
if <(sprite 2 y pos) > (sprite 1 y pos)> then 
go to front
end
end

Thank you everyone for the help!