Is is okay to wear APFU to my high school if I am conducting PT in JROTC later that day?

AR 670-1 "The physical fitness uniform is authorized for wear on and off duty, on and off the installation, unless restricted by the commander. Soldiers may wear all or part of the physical fitness uniform with civilian attire off the installation, unless restricted by the commander."

Would you go to work in your gym clothing if you were going to the gym later on that day? In my opinion, wearing the apfu around as civilian attire looks very tacky. It looks extremely casual and I don't think it presents a very professional image to be at work or school in it. On the other hand, it would be great for you to wear it during PT.
Remember, that when you are around civilians and sporting your army gear, you are a brand representative. You are representing the Army brand

Check with your JROTC leadership and/or your cadet handbook.