Dear Evil Editor,
Seventeen-year-old Andrea Smith has just witnessed a murder—and her close friends committed it. Her friends decide [decided] to play a prank on a new student at their high school. One of the popular girls in the group pretended to be his girlfriend. He finds [found] out about the prank and a struggle ensued with the girl
Andrea tried to break up the fight but was unsuccessful. Before she had a chance to tell the truth to the police, they [her friends] beat her to it and lied instead. [Not clear why this would prevent Andrea from telling the police what happened. The police assume some people will lie and some won't.] Andrea becomes [is] devastated. It's like she is the only one that [who] has a conscience. This causes her to distance herself from them [her friends]. If that's not bad enough, they refuse to show any remorse, especially her long-term [time] boyfriend. Later, Andrea breaks up with him and he becomes insanely jealous when she meets a new guy that [who] shows her the affection that he [her boyfriend] never gave her. [I'd reveal the boyfriend's name so you can use it when he comes up.]
As she cut [cuts] ties further with all of her friends, she [Andrea] develops a close friendship with the dead classmate’s sister. It becomes at risk when Andrea keeps her in the dark about what happened. Andrea doesn't want [, not wanting] to hurt her even more and she tries to figure out a way to tell her the truth but she can't bring herself to do it.
Now, Andrea’s been noticing that someone is stalking her every move when she starts receiving death threats. She began [begins] to wonder [and wonders] if someone is enacting [exacting] revenge against them for the murder. Is it the sister that [,who] found out some kind of way [somehow,] or someone else. [?] She becomes terrified when the stalker [someone] starts harming them [the friends] one by one. Andrea needs to get her life back on track before the sins of the past come back to haunt her. [She needs to tell the authorities what's going on. She can get her life on track later.]
CRUEL SACRIFICES is a 51,000 word YA contemporary novel, with thriller and mystery elements. This is a standalone novel with series potential. Thank you for your time and consideration.
It seems Andrea would suspect someone is out for revenge after the girls get harmed one by one, not after she, who had nothing to do with the "murder," gets death threats.
Not sure why we're referring to the "murderers." You declare that a struggle ensued with the girl, that led to his accidental death. If he confronts her and they start shoving each other and he falls down, hitting his head on something sharp and dying, they aren't going to charge all the girls with murder. I doubt they could even get the one girl for involuntary manslaughter.
You start with backstory, which is okay, but you switch from past to present and back. When you get to the actual story (Andrea becomes devastated....) you need to stay in present tense the rest of the way, but you again use both past and present tenses.
Too many character names can be bad, but you name no character except Andrea. The others are her friends/the group/the murderers, her long-time boyfriend, a new student/the new guy/the dead classmate, the dead classmate's sister. One or two more names of key characters wouldn't hurt.
Pronouns should be used only when it's clear which noun they're replacing.