Why do teachers do this

I press "k"

i too

I was about to comment “literally every teacher” when I saw the title

Some teachers just take 10 seconds to exit full screen then another 10 seconds to pause

yes

Yet teachers every time they have to pause anything.

Tf do you mean by the Lil button in the corner?

true

Spacebar is large and easy to press.
"K" is small and easy to press.
The mouse might be in the wrong place so you have to move it and it's harder to press.
The button in the corner takes the most energy to press.

I do it with the k key