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We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
Thirteen-year-old Nova Labelle knows eighth grade is going to be perfect. She will get straight A’s. She will start dating her prince charming. And she will continue very slight awkwardness in phrasing here - not bad at all, but "continue to be" makes this sentence longer than the ones before it, and messes up the rhythm of what you're trying to accomplish here to be school royalty. There’s just one humongous I'd consider striking "humongous" for the same reason, rhythm problem: that new girl, Stacy Winter.
Nova has always been the most popular girl in school. She’s got the looks, the brains, and the popular girl personality down to perfection. I think you established all of these things in your hook (and did it well). You're just repeating yourself here. Use this para to focus on the List She’s even remained at the top of the List, a piece of paper that keeps track of the top seven popular girls in school. Nova consults the List every day to make sure her rank stays where it belongs. But Stacy seems to have a knack of mercilessly destroying every single one of Nova’s eighth grade plans.
Stacy is threatening to steal all that Nova has, from her top spot on the sacred List to her future husband. She claims she has no reason to become the next Queen of Cadbury Middle School, but Nova knows better. Everyone wants to be number one; everyone wants to be her. So Nova’s got to guard everything she’s ever come to care about.
But the thing is, Nova’s life screw-up-er might not be Stacy after all, but something that lies in her own reflection.
WHO’S THE MOST POPULAR OF THEM ALL?, complete at 34,000 words, is an upper MG contemporary novel. I would describe it as a Mean Girls style retelling of Snow White in the POV of the Evil Queen.
Overall this is fantastic. You've got a great hook, establish a wonderful MG voice right off the bat, and you even have a catchy title. I'd clear up a few of the awkward parts and the slight repetition and then get querying!