Seventeen-year-old ANDREA SMITH is smart and sophisticated. She has a wonderful and supportive family. She has a boyfriend named TONY. She hangs out with the most popular, coolest, and the wealthier crowd at Riverdale Heights High School. Some of her closest friends decide to play a prank on the school's biggest nerd CODY by getting one of the female friends to pretend to be his girlfriend. But it isn't a secret for too long because the whole school learns about it. [You never said it was a secret to begin with.] [The whole school including Cody?]
Including Cody's sister SHANNON who became a new friend of Andrea's before the prank and Shannon's boyfriend FREDDIE. [That sentence should be in the same paragraph as the previous sentence.] Cody accidentally gets into a fight with the girl after she publicly breaks up with him at a rambunctious party one night. [You said the whole school knew about the prank including Cody's sister Shannon, so I assume Shannon would tell Cody, and the lovebirds would be broken up before this party.] She pushes him and he slips on the concrete, hits his head and dies. Andrea and the others panic big time by covering up the crime and lying to the police. Then the friends decide to take an oath by burying [to bury] the truth.
Andrea feels guilty but Tony and the rest of her friends could care less unless they get caught. Andrea begins to distant [distance] herself from her friends. Tony is showing less affection toward her and this causes her to look elsewhere. At a gathering one night, Andrea meets an older college guy name LEE. She becomes drawn into this new relationship with Lee after she abruptly breaks up with Tony. Tony learns about their relationship after gossip about it spreads around school. Tony starts to taunt the two and decides he wants Andrea back no matter the cost.
Andrea decides not to get back together with Tony. She discovers Lee is a mysterious individual that [who] carries many dark secrets. Andrea and her friends began [begin] receiving threats, a dead animal corpse in a mailbox, blood in a bed, followed by a shooting that sends them over the edge. Soon, they realize that someone else must have witnessed the crime [The "crime" occurred at a rambunctious party. There are probably a dozen witnesses] and was [is] planning revenge. In the news, everybody learns of a killer roaming around.
They believe that maybe this killer is connected some kind of way after an individual claiming to be the killer taunts Andrea online. Two of her friends disappear without a trace and is [are] later found dead. Andrea and her friends soon realize that this killer is stalking [them] and later starts killing them one by one. They can't turn to the police because [they took an oath.] the police don't believe them. [Four people who were involved in Cody's death have been killed, but the police don't believe there's a connection?] So they are left to fend for themselves. Andrea finds out that instead of one killer, there are two killers.
She learns that Shannon and Freddie have teamed up to avenge Cody's death and they murder the rest of her friends. Andrea escapes and calls the cops.
[Andrea: Ten of us were present when Cody died, and so far nine of us have been murdered.
Police: Probably a coincidence, but if you get murdered too, I may rethink that.]
Shannon and Freddie catch up to her and trapped her. Shannon tells Andrea everything that she and Freddie did. Shannon turns on Freddie and shoots him in the heart, killing him instantly. [Why?] The cops arrive and see Shannon pointing the pistol toward Andrea. They yell at her to drop the pistol, but she is oblivious to them. Not wanting to suffer a life confinement in prison or get the death penalty, Shannon turns the pistol on herself.
A month passes[period], Andrea's new boyfriend Lee drives up to her house to see her after weeks of her avoiding him. He wants Andrea to rekindle their romance they once shared. Andrea reveal[s] to him that she wants him to give her space for now because she wanted [wants] time to ponder her life and commit to celibacy. He sadly acknowledges her choice and leaves. A friend calls her cell phone[period], she turns down a request to go out and have fun. Andrea begins to daydream as she watches the clouds in the sky. [You can do without this paragraph.]
Lots of errors, mostly switching tenses.
The police and the kids all seem pretty stupid. Cody slipped on the concrete. His death was an accident. Now all these kids are covering up the "crime" of pushing Cody, which was committed by only one person, and two other kids are committing numerous premeditated murders. I'm sure someone in the book is the voice of reason at some point, preferably your main character, so make that clear in the synopsis.