If I understand you correctly, you are trying to say that teachers should not prepare students for something they may come across in real life (rather than the idealised one,) Hmmm.
Were I a teacher, I would have a problem with that mentality. I think 'forewarned is forearmed' applies in the case of knowing about an expression that may be occasioned upon in both speech and the written word. Surely to 'educate' is to impart knowledge, not restrict it to that which is deemed proper by those who should know better.