Our district had to make that decision three years ago for the very same reason.  It was quickly decided to expand the elementary school and move the 5th Graders back to the elementary building.  It made many of the parents uncomfortable to know that the 5th graders were co-mingling with the middle schoolers.  It is already such an impressionable age and while most of our middle schoolers exhibited good behavior, there are always a few bad apples.  The kids didn't like it either.  They were sort of lost in a sea of middle schoolers and weren't quite ready for all the bustle of changing classes and moving around so much.  The addition is finished now and the 5th graders are back in the ele. building.  I don't know about the law where you are, but they should do some research before making such a dramatic decision.  Elementary ed. teachers are usually certified to teach k-8 so there most likely would not be any violation of law.  If they move forward, they should consider keeping the 5th graders isolated to one area of the school, and choose not to have them move every hour.  I think the teachers will do fine, it is the social aspects that are of concern where the students are concerned.