I grade all student work at school unless I am pressed for time like at
the end of a semester. I call each sudent up one at a time to return work
(while they are working on a new assignment) and make any comments
personnaly at that time, quietly. I have once per week checks of shetch
books and generally suggest future pictures based upon their sketches, I
do not critique sketches because they are for brainstorming and not
improvement in technique.