[vegai_]

>there were a number of classmates whom could not grasp

Sorry, pet peeve. It's "who". "Classmates" are the ones doing "could not grasp" instead of "could not grasp" being done to them.

[UncleSam]

Any easy trick I use to help remember which to use is to replace who/whom with he/him to see which makes more sense in the context.

"He" could not grasp. => "Who" could not grasp.

I should vote for "him" => "Whom" should I vote for?