Mina Bevins hasn't leaked any magic like the other girls, earning her the status of weirdo in the world of witches. When her thirteenth birthday passes, and she still lacks any power, the Coven Seer looks into the past for an explanation and discovers Mina’s not really a witch at all. She was switched at birth with another baby in the mortal world hospital.
Somewhere there’s a young witch leaking magic all over the place, exposing the witch world. Since secrecy is essential to keeping the coven from persecution,This is the weakest paragraph to me. I love, love, love that her reaction to finding out what they plan to do is to rescue the girl, but your voice switches ever so slightly here. Your first two paragraphs are whimsical and show strong worldbuilding (leaking magic, mortal world hospital, etc) and that's lacking here. How would Mina tell us how she's feeling and what she plans to do? Try rewriting this paragraph in her voice and then playing with the pronouns and such to make it work in the body of the query. Your synopsis starts off so strongly and I want to see it end as strongly.
andthe only way to ensure it remains safe , the coven decides,is to track this girl down and remove her powers. Having just learned that she will never come into the power she so desired to possess, Mina thinks stripping someone of her magic on purpose is horrible. Together with the help of her friend Porter, a male witch, Mina uses the limited power available to them to find the other girl and warn her before the coven takes away her magic life too.
I love this title so much.
At 58,000 words, sWitch is middle grade light fantasy that brings to life a matriarchal world of witches hidden among humans. (personalization to agent)
Thank you so much for your time and consideration.
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Most of my comments are totally nitpicky, but this query is close to perfect. A tweak here or there and a stronger closing paragraph in the synopsis and you're set. Good luck!