I think you are in uncharted territory. I will ASSUME (we all know how that is spelled) that you are referring to a "Classroom BoB" in the event a "Lockdown" would be required.

Here are my thoughts (shelter, water, heat, food):

1) (water) At least one 20oz bottle of water per student + teacher
2) (security) The means (eg: Mechanic's wire) to secure all classroom doors and windows from the inside.
3) (entertainment) One deck of cards for every 4 students and perhaps a board game for every 4 students.
4) (hygiene) Select a location in the classroom that a temporary bathroom can be established using a couple of dark sheets to provide privacy, 13 gallon trashbags to line a trashcan to make a porta-potty and 3-4 rolls of Toilet Paper, and a large can of Lysol disinfectant.
5) (communications) A cell phone (if the teacher/students are not allowed to have them in the classroom) with ICE numbers for help (911), and any other pertinent numbers.
6) (medical) A FAK with emphasis on trauma type dressings.

This ASSUMEs a great deal; that a Lockdown probably will not last more than a 12 hr period, and is primarily some violent type of threat on campus that the friendly, local, neighborhood SWAT team is going to be responding to and cleaning up.

In regards to having a BoB for other situations, the "buddy system" would probably work best in the case that your students live on campus, I would suggest a BoB for each Dormitory room and the occupants are responsible for procuring the items and maintaining the BoB's. These would be comprised of standard 72 Hr kit items for two persons.

If the students live at home, then I would recommend handing out the standard USA Gov't preparedness info and leave those decisions (or lack thereof) to the parents,individual households.

I recommend that we work this particular problem to a realistic solution that you can apply in the real world and then we could tackle the question of a Teacher's BoB.