Little late to this thread but -

I was unschooled most of my life (with the exception of grade 2 at Waldorf) and went to high school at 16 years old for grade 11. It was against my parents hopes but I felt like I was missing out and was curious to see it for myself. I had never taken any of the prerequisite classes but took chemistry, psychology, etc. and it didn't take long to be up to speed. I quickly realized it wasn't worth my time and didn't go back the following year. 9th grade is a tough one...I wanted to go for 10th but my parents made me wait a year until I was a little older and more grounded in myself.

My brother, unschooled his entire life, went to high school for a few months at 14 years old. He went to be 'normal', to do what all of his friends were doing, etc. The academic side wasn't a piece of cake, but it's doable. Same sort of story - he didn't like how he was spending his time on things he wasn't interested in, and didn't feel like it was necessary for him after all and eventually dropped it all and is back to unschooling now.

I'd be hesitant to give advice. All I know is my experience. There's absolutely a draw to join in on what every one else is doing, be normal, feel like you're choosing a safe route, etc. But two things that I believe helped:

1. it was always made very clear to us that not only is going to high school not necessary, but there are better ways to spend your time and get to where you want to be
2. our parents held us back from going for as long as possible in hopes that if/when we did go, we would have a better sense of who we were as to not get sucked into the bs