It is very easy and makes so much fun, just let the people know...

Idk. 

Let the students choose which subject they want to grade. 

Make graded homework more lucid.

Send messages to student who reached level 3 that grading homework is now available.

Teachers could post which students graded the most homework and reward them.

When homework becomes graded faster maybe it will make doing homework more attractive -> bonus!

What ever - just do anything about it.