As a former school superintendent and a middle school administrator, I would like to share a reply regarding your question.  It is not unusual to find middle grades schools covering a wide range of grade spans- 4-8, 5-8, 6-9, 5-9, etc.  That is why the term "middle grades" is often used now instead of middle school.  If I had a child in a middle grades school, the #1 issue is the competence of the teachers my child will have.  The grade structure is totally secondary to the quality of the teachers.  Give me good teachers every time and all the rest of it is secondary.

That said, another major issue as a parent is that of safety.  Once again, I don't think you'll find any one grade configuration that guarantees safety, but if your concerned I would sit down with the principal and ask him/her to outline in specific detail the safety measures for those 5th graders.

Hope this helps!!