Concerto Competition

I'm looking for a piece to play for my high schools concerto competition as a student next year. They usually look for traditionally classical stuff and we are unable to have a band/orchestral accompaniment (so either piano or unaccompanied) , but aren't super strict on the traditional classical concerto part of it. I've been looking to find a piece to play (6-10 minutes long, can be select movements), but it's been difficult for something I really like in my skill set. I've looked at some common ones, and Fantasia (Villa-Lobos) has been an option, but aside from the tricky part in the 3rd movement I don't know if it'll be impressive enough to win the competition (we do a short section as an audition then the winner(s) perform the full thing at the concert). Any advice or favorite pieces that I should consider? I'm still open to both soprano and tenor pieces.