you dont really think they'd bother to check do you? even if they did: 1) couldn't prove it wasn't you had written it in the first place, and, more importantly, 2) ive never seen anything on this site that has any new or radically controversial ideas. the sort of stuff people generally say on this site is a general body of critique. it is not considered necessarey to quote generally accepted ideas (unless you want to) as it is accepted that they are not the 'intellectual property' of a particular critic. any new interpretation by a critic, though, must be appropriately quoted.
so, basically, no. if someone puts something in a way that you could actually use in an essay without changing (which i seriously doubt anyway), than it would not be considered plagarism by your teacher.