Outside of the occasional training course (as a student) or in-service training (as a trainer), I'm no longer an active participant in the education game. Therefore, apologies in advance if what I say lacks relevance to your situation.

If you work off lecture notes, then that suggests at least a couple recurring NAs: a daily NA to update today's lecture notes and another NA to review tomorrow's lecture notes. However, I am guessing that the bulk of your professional work comes in several big chunks which you have to wade through quickly - the chunky activities being things like grading papers, exams, and the like. I don't think you need to create NAs for those activities as you are unlikely to forget about doing that sort of thing. That is, unless you are the sort of person who REALLY enjoys checking things off lists! Then, by all means, include this stuff as an NA or project (more on the project aspect later).

In my opinion, you only need a NA for things that are crucial to your well being. Things that you just can't afford to forget about or for those activities that you worry (or might worry) about. One key benefit of GTD that is easily overlooked is the importance of capturing "distractions" for future processing. Not only does capturing the idea for deferred action allow you to get back on task, but it also helps you cope with large tedious tasks (like grading exams or papers). How so? If you are on your 3rd or 4th note in a ten minute span, that is a good indicator that your brain needs a break, and it is already taking that break on its own!

What do projects have to do with grading papers (or any lengthy task)? It is an activity that by sheer volume can feel overwhelming, and it can be tedious as well! Therefore, make a project out of it, and dynamically break it down into pieces that are appropriate to your context, energy level and the urgency of the situation. I tend to go for larger bites - right at the edge of what I know I can do. Not only can I rely on my competitive streak to help with focus, but it serves as a good reality check. If I'm off my game, at least I'll have to acknowledge the fact, and deal with it.

Do you mean, literally, in the classroom, referring to classroom activities, teaching, etc?

I think it all boils down to the next-action part of the GTD model. If you want your students to write a paragraph, break it down into tiny steps.

From my end (university teaching and grad school) the problems are more related to organizing marking and correspondence with my students.