I had some serious incidents in High School, mostly with other kids, and while my grades dropped, I never lost the ability to make up for weeks, sometimes months of doing nothing for school in a matter of hours.

Didn't stop me from getting bad grades for those weeks/months and making stupid mistakes like not completing assignments and the like.

It's very easy. It's just that in a depressed/anxious/... state people aren't capable of sustaining even the most basic levels of effort after a while. Frankly, I bet that if it was a lot harder, that would have helped me.