one enraged girl, barely more than a child, hurls a stone at the hated invaders...and effortlessly with magic she didn't know she had penetrates their magical shield

[Kate (Feathered Turtle Press Reviews)]

Sounds kind of like the description on The Magicians' Guild:

This year, like every other, the magicians of Imardin gather to purge the city of undesirables. Cloaked in the protection of their sorcery, they move with no fear of the vagrants and miscreants who despise them and their work—until one enraged girl, barely more than a child, hurls a stone at the hated invaders...and effortlessly penetrates their magical shield.

[Kris]

Liv, here's the Google Books preview of The Magicians' Guild by Trudi Canavan - KB's suggestion:

https://books.google.com/books?id=cNw... There are different cover images.

Should we mark your request as Solved?

[Lobstergirl, au gratin]

Liv, please add some plot details to your header/title. This could help anyone in the future searching for the same book.