Don't think you could guarantee being the only Kitty, Freya, Darcy, Thea or Tegan in a class.
Callie - quite like this
Liberty/Tigerlily - not really a fan
Mimi - quite sweet but isn't it a word for grandmother? Maybe as a nickname for something? Just in case she wants to be a judge or the prime minister or something (sorry for the cliche)
Fenella - like this, reminds me of an Enid Blyton book set in a circus.
Ailie - Like. Prefer it spelled Eilidh but realise that this looks odd outside of Scotland and within Scotland is very popular.
Lydia - Like, don't think it's too common
Kyra - doesn't do it for me
Octavia - probably will be the only one but personally feel it's a bit poncey
Oonagh - like
Hanna - not as popular as you'd think.
Quite a lot of pot luck though, so pick something you genuinely like. Even an unusul name could have multiples in your school, conversely a popular one could be the only one.