Thanks to Kathryn for creepy treehouse. There is a great definition of the term by Jared Stein, and as I read it I realised that this was a term we in the education industry need. It is said to have been coined by Chris Lott in January this year, and to be current amongst "ed techs".
The definitions in Flexknowlogy (above) are "n. A place, physical or virtual (e.g. online), built by adults with the intention of luring in kids." You can substitute the words teachers and students for adults and kids, or for that matter anything similar - lawyers and laypeople is another. And
"n. Any institutionally-created, operated, or controlled environment in which participants are lured in either by mimicking pre-existing open or naturally formed environments, or by force, through a system of punishments or rewards." There are more, and Mr Stein returned to the topic later.
My own non-educational example is the possum box that Peter the Possum Man installed in an old pear tree after he had removed the possums from the house roof, to their disgust. The possums climb past it every night, but never use it. Too creepy, I guess. Is that like a teacher setting up a Facebook group for the class? Or not?