I just graduated high school but by the end of my junior year, I knew exactly what I wanted to do following high school. I would be going to a local university and getting my Business Economics degree. This might change but I'm glad I had an idea of where I was going and what I was doing. Like a literally did not have a backup choice. It was this school or nothing (which I got into so I'm happy about that).
But I don't know why you need to have a solid plan of what you're doing after high school. Your brain isn't fully developed yet, and we still receive so much pressure to choose what we want to do for the rest of our lives. I believe you should have an idea by your senior year of whether you plan on going to college, joining the workforce or military or going to a trade school. But you shouldn't have to make a full, set-in-stone decision.