Hiya Dani,
Yeah, friends tend to be nice. We know better here on the board.

Here are my two cents - for whatever they're worth.
--------------
Jeanie Thompson has gone from popular Florida beach girl, to friendless country hick. Banished to live with her grandparents in tiny Fillmore, Missouri, she spends each tedious day alternately sleeping and being force-fed three square meals by her disapproving grandmother. --- WHen I first read this paragraph, I thought it was fine. After reading further, I see that some accident was responsible for the move. As long as you're going to talk about the accident, this is the place to mention it first.
In spite of her currently miserable existence, --- You've already shown that she's miserable, so you don't need to repeat it.
Jeanie notices a strangeness about the Fillmore residents. --- give some BRIEF examples.
At first she writes off their distinct hostility toward her as her own paranoia, but when the ice cream truck “accidentally” swerves and almost hits her, she begins to realize that her suspicions are anything but imagined. --- Wordy. Consider: When the ice cream truck “accidentally” swerves and almost hits her, she realizes that her suspicions are anything but imaginary.
When she confronts her childhood playmate and town golden-boy, Pierce Adler, --- but if she is from Florida, how can he be her childhood playmate?
Pierce has learned about the tragic accident which forced Jeanie's relocation to Missouri, --- Mention the accident in the beginning.
and suddenly it make sense why everyone in the town has been staring and whispering about her. --- It doesn't make sense to me.
After befriending David Dresden, the brain-damaged driver of the ice cream truck, --- How does someone with brain damage get a commercial driving license?
Jeanie decides that the cold shoulder she's received from Pierce's popular friends, along with almost everyone else, is about something else entirely. --- What?
Especially when Pierce, who constantly pops up at inopportune times, swears he never told anyone her secret. --- What secret?
If it's not about what happened in Florida, then why are the residents of Fillmore after her? --- This sentence doesn't tell me anything about the story.
After a second near-death experience --- consider 'a second near-accident'. Since the first wasn't described as a near death experience, using that phrase for the second is confusing.
and learning that her new friend David was also a victim of the town's malevolence, --- the near drowning? If so, mention it here, not later.
Lissa Mortes, --- that name means Death. Is this deliberate?
Due to the influential connections of the kids involved, it was allowed to be over-looked as an unfortunate accident. --- Confusing. A few words about rich kids being suspected in the near-drowning would help. "...it was allowed to be over-looked...' is vague and passive - not a good combination.
Fillmore, with its elite founding families, including the Dresdens and the Adlers, is run quite differently from the rest of the world. --- Too vague.
Relieved to have an ally in her quest, Jeanie believes she can figure out what's going on and get justice for David. --- Wordy and vague. Consider: With an ally's help, Jeanie hopes to solve the mystery of the town's behavior, and get justice for David.
That is, until the night she walks past the abandoned lodge, and witnesses what appears to be a sacrificial ceremony. A human sacrificial ceremony. Performed by white-masked, white-robed freaks. Jeanie is about to run for help, when she is intercepted by Pierce, who refuses to let her tell anyone what she saw. Realizing that she has no proof, and that no one in the town will listen to her anyway, she is forced to keep quiet about what she saw. Jeanie is in way over her head, and the evil permeating Fillmore extends far beyond a group of popular kids getting their way. --- Have her at least check the news for reports of missing persons. Consider: One night she walks past the abandoned lodge, and witnesses what seems to be a human sacrifice performed by white-garbed freaks. Pierce intercepts her and warns her to shut up about what she saw. Since she has no proof of what she saw, and doesn't have any credibility in the town, she maintains her silence. The evil in Filmore extends far beyong a group of spoiled teenagers.
Pierce and Lissa, each warning Jeanie about the other's intentions, means Jeanie doesn't know if she can trust either one of them. --- Pierce was a collaborator in the murder. Of course she can't trust him. I suspect you can do without this sentence (besides which, it's wordy.)
Both Pierce and Lissa let more details slip, including a story about the first true founding family, The Storms, whose descendants disappeared without a trace some years ago. Jeanie is desperate to put together all the pieces, but none of them seem to match up. --- Consider: Digging for details, Jeanie learns that all the descendants of the town's founding family, the Storms, disappeared some years ago. Jeanie can't put the details together into a coherent picture.
Finally, the night of the Founder's Day Carnival, everything begins to fall together. Jeanie discovers disturbing evidence that Pierce was involved in every horrifying event she knows about. She turns to the only friend she has left, Lissa. But when she finds herself in the middle of the same ceremony she witnessed the night at the lodge, she uncovers what a huge part of the story the Storm family is, and that at least one member of the family never left Fillmore at all. Suddenly Pierce's betrayal isn't the most shocking one of all. --- The night of the Founder's Day Carnival, Jeanie learns that Pierce has been involved in every horrifying event she knows about. She finds herself in the middle of another human sacrificial ceremony, learns that one Storm member is still in Fillmore, and realizes that Pierce's betraying isn't the most shocking one.
------------
And end with what the most shocking betrayal was. I agree with the comments about about a synopsis not being a query, and not having a cliffhanger ending.
In summary, I think you most want to work on being more clear in what you're saying and how you present what you're saying, and in cutting out non-essential wordiness.
Hope this is useful. I have a synopsis on my novel 'A Virtual Affair' on the board. If you could review it, that would be great.
Zvi the Fiddler