I dealt with this first hand! When I was in 7th grade I ended up becoming friends with the 8th grade girls. The girls my own age and girls that I considered friends for years blew me off because I had a different teacher. This was a California school in which you didn't change classes each period. You just had one single teacher all day. I never saw my old friends until lunch time or after school and they wanted nothing to do with me.
So I had to find new friends... and it ended up being older girls. The "cool" girls!
I have to say looking back on it, it was horrid for me. However at the time I thought it was great and felt all special and did all these grown up things (so I thought) but in reality it put me into a world I was not prepared for. I went to parties with drinking, boys (sex), drugs ect...
I knew the whole time it wasn't right and so wasn't me, but I wanted to fit in and I thought these girls were SO cool. Of course when the year ended and all my new friends went on to high school I was forced to make friends again with my old friends and that was hard. But I was forever a different person. I was a pretty naive girl and saw and did things that I hope my young daughter will wait until she is much older to experience.
While I don't think older kids are bad as a whole, it is something to be concerned about that they are friends on any level. At that age, unlike when we are adults, there are major differences in who we are and what we think. They base things off peer pressure and the desire to be liked, more than common sense and doing what is right. For some reason at that age doing things we know we shouldn't seems so appealing and that urge is hard to fight!
I am not sure how you can control things and I hope you get some good advice from other moms, but coming from someone who lived this, I would say you are right to be concerned.
I wish you luck in this struggle. But I wouldn't just ignore it. While I made it through it all with only minor emotional scars, not every kid is so lucky. I had a good family that snapped me into shape and I went on to live a life completely opposite from that all. In a way that bad time was good for me in the long run, but had my family not been there, I am not sure I could say that today.