- Money (Especially if a functional lab and supplies are needed; I saw one DC private school with a 'virtual lab' for dissections etc)
- School size (Enough kids to fill the class? Some schools won't run a class with fewer than 8 kids, for instance)
- Student preparedness/interest/parent demand (Which is really money, in a different outfit)
- Equivalent options (Is it easier to send the kids 2 blocks over for a DE class?)