Well, the first one is not relevant to any of the sites, the second two are relevant as they deal with the StackOverflow platform.
This question is aimed at teachers, but it could also reasonably be considered a question for the SO community about the acceptability of teachers sending their students to SO for their questions. In fact, that question already exists. So clearly it is an appropriate question for meta, because it is about what the community response should be to an evident behaviour.
As I mentioned in a comment on a different response. This question is also appropriate for MetaSO because it is about knowing your users. Everyone on S[O|F|U] is providing their time for free, and they are getting... some... reward. Some people do it for rep, some for fun, some for whatever, but the point is that each person has a reason to sacrifice their time. And to someone who may or may not be intent on building their own StackExchange community, or someone who is just wondering about the community, would have to ask that exact question. And it would belong on this site.
Although, for an answer, Jeff has already blogged about it, when he talked about Micro-payments, and the Experts-Exchange model both being insufficient for StackOverflow. Once again, the question is appropriate here.