I recently had the question asked: "Why do we 'separate' middle school and high school and isn't this a form of division in the church." Below is my response to the question.
As a father of a pre-teen and a youth pastor for almost 18 years I love having my daughter around peers while she is "away" from her parents. For me it is not about the economic status or maturity level of students, but rather about the environment which she is in.
We had an all student ministry beach day about a year ago and an incoming 6th grade girl was dropped off by her father. After parking the car and opening the door the car was surrounded by many students who were excited to "welcome" this new student. In the crowd was a couple shirtless, facial hair sporting high school guys greeting her as well. Her father decided that an "all" event would not be the "best event for her to start with" and took her home.
As a student pastor for many years I was able to instantly able to think of many reasons why he should have left her; relationships, opportunity to build trust, the community that would take place, leader connections for future investments, etc. HOWEVER, as a father, I couldn't think of one. A safe environment that involves building relationships with peers who are in a similar developmental stage physically, mentally and socially is very important. I am not opposed to some ministry events and programs being combined if they are VERY intentional about the why and how to ensure that students needs are being met and it's not just a "kill two birds with one event" approach.
Jesus had 12 that were close, and 3 that seemed to be closer. I don't think we have separate age group ministries for the sake of separation, but to meet students, and adults where they are. We can't give solid food when they are only ready for milk... and visa verse.
Student ministry is difficult. I find it more sustainable, relevant, and healthy when we are able to speak to students and parents about the stage of life they are in and encourage them to make healthy choices in the middle of their everyday life that reflect Jesus.