For most people the high school years had a major impact on your life. For some it was about great social and athletic success or challenges. For others it was about academic achievement and important friendships that shaped your character or anxiety about the possibility of failure or social isolation. For most, it played an important role in how you functioned in dating relationships.
With so many issues tied up with those years, it is no surprise that day to day event years later may evoke thoughts or dreams about similar issues and the dreams may be set where those issues first seemed important in your life.
For those who spent 10, 15 or even 20 years in University, that setting may eclipse the “good old high school days” as the setting for dreams.
The significance of dreams for most people is the content (what they are about) more than the context (where they are set).
The issues that may be still unresolved may push their way into your dreams must more than things that are not that meaningful or important to you now.
I don’t advise anyone to worry if they still have dreams set in high school. If you can identify the important issues and figure out what they mean for you now, they can still be helpful to you!