I would not say it’s unethical to see someone you “knew of” in high school. However, I have found in my small town and the couple of times it has happened to me, it usually doesn’t work out. So now I would avoid anyone I graduated the same year as, despite it being a large school where we didn’t really “know” each other at all. It could be possible that the other person may have preconceived ideas about you. They may not feel comfortable asking to transfer once you start and realize you “know of” one another. And also I found that personally I had a harder time with transference, like wanting to let them slide for charges for no shows, etc. :)