Mom of a teen girl here.
It's very late in the school year-- has this clinginess and constant contact with the HS friends gone on all school year long? Or has it increased as the end of the school year is approaching? If the latter, then your daughter may be feeling unsure and insecure about leaving MS and going on to HS. But if this has gone on all year, and especially if your daughter has not made her own friends in MS because she's only marking time until she "catches up" with her old friends--that's a bad sign. She is setting herself up to be very hurt if she gets to HS and finds her gang there has lots of other friends and hasn't been waiting and pining for her as much she has been for them. She may find that she is only one friend among many, for them, when she turns up at HS expecting them to be her entire social world. That isn't going to happen and she'll be let down.
Does she have any extracurriculars? Something in school where she has met some kids? Can you suggest she ask one or some of those kids to do something outside school? Even better would be if she has an activity unrelated to school where she meets kids who share an interest of hers-- does she have such an activity? And what is she doing over the summer? I would steer her to some structured activities based on her interests so she is not just hanging around all summer trying to see the old friends. Yes, she should keep up with old friends but she needs a wider world than that, too. Work with her to find something that is not merely social--for example, if she's into arts, there are great art or drama camps for teens. If her thing is computer coding, yes, there are teen workshops in summer for that....let her feel she is getting the power to make a great, fun choice. She may not make friends who last forever but she'll learn there is more to do than pine and text.
When school starts, be sure she gets involved right away with some interest-based and service clubs at school.
Clubs and camps aren't the answer in themselves but the idea is that she needs to get outside this tight circle. Do NOT say anything negative about her old friends, though, and do not say that you want her to do activities in order to meet new kids; that will only make her defensive and resistant, understandably, as she will feel you're pushing her away from her friends. You're actually not doing so, but expanding her circle, but she has to play a part by trying some new things.