I could never believe that there is are colleges out there who would not admit for your lack of experience. You go to college to learn, to acquire, to grow. I entered the theatre arts program at UM as a non-conservatory student which meant I did not have to audition and am free to explore what I would like to do most. If I ever made the choice of going into stage management, all I have to do is take the classes and participate in the shows. In your situation you want to go head on into stage management. And there are conservatory programs for that which would require a portfolio. I imagine a portfolio would consist of a few prompt books from highschool and letters of recommendation. But you initially go to college not to build on what you already know generally, but to acquire the knowledge and skills neccesary to do what you want to do. No highschool, unless magnet or that focuses heavily on the arts can prepare you for that.
Definitely contact the school that you want to go to and find out what you need to know, since as of now you're simply speculating. No one can know everything, especially as a student. best of luck.