Not likely the Killer: Noah, Jake, Brooke, Piper, Mayor Quinn, Maggie, Sherriff Clark.
Each of them were with somebody at the time of the murders that took place and victims who became survivors.
Considering Jake was stabbed, and I doubt he's brave enough to do it himself. I was hesitant to add Audrey because, I have a hard time figuring out if she was far enough away or with somebody to rule out her being the murderer. I can only think of the time when she was with Noah, and showed up to hear about Riley. And I believe she values her friendship more with Noah, and wouldn't have it out for Riley for any known agenda. Including the fact she wouldn't kill her own girlfriend Rachel. (Even though, they weren't exactly girlfriends with each other...) Then there's Mayor Quinn. He is a bulky person, who wouldn't be able to walk around silently or do the job himself - And he was behind bars when Brooke was attacked.
Random Note: Audrey hasn't been chased after or explicitly been on the Killer's hit list either. So, I'm not going to add her until I feel like she has enough to back her up.
The Deceased: Tyler, Nina, Rachel, Riley, Will.
The Remaining Suspects: Kieran, Mr. Branson, Kevin Duval, Mystery Child, or Two Killers? Brandon James (who may be deceased?)
Two Killers: I like how it was mentioned there could possibly be two killers instead of one, because there's a lot of spying going on in the show. Therefore, somebody is really good at technology to place the timing just right to kill somebody on their own, or has another party to keep tabs on Emma and everyone she's involved with to alert the 1st killer to execute the plan. As well as goad Emma further. And it seems like Ghostface sure did his/her homework about Brandon James. Including the history involving Emma and Maggie. Which they either had to know from another person, or searching for birthparents after being adopted?
Mr. Branson - He could likely be in cahoots with another killer. What with last night's episode revealing his identity is fake, he seduced a female student, and is suspected of murdering her. What would be EXTREMELY CRAZY is if the female student was ALIVE and found out she was related to Emma? [Since we still don't know the gender and this would become plausible for a few other reasons. 1. Ghostface's body looks a little bit slender to me, plus the short arms and smallish hands. I feel like the attempt on Brooke or Noah with the knife came out short for that reason. 2. We don't know for sure if the female student is dead unless I missed that part last night, because I think they said the girl was missing. (I may of had a third thing going on this, but I forgot it.)] However, the only real problem with this could be likely hood of the missing/dead girl from the other high school is literally dead, and the ability to carry/string up somebody to a ceiling fan. (I imagine a guy would have a easier time doing this.)
As for Mr. Branson himself, I do believe he's framed by Ghostface to throw off the police for the time being. Awfully convenient to hide evidence inside of a classroom and time it right to make it look like Mr. Branson hurt Brooke. Another thing is that he could fit the bill. Some guys do have slender bodies. I could've also misjudged on the length of Ghostface's arms. And he wasn't there to hurt Brooke, but he knew where she was staying. So this guy could be Ghostface or part of the team to execute the plan. And it was mentioned by Noah about it being possible he needed to kill those with access to his laptop for mysterious reasons too.
Kieran - Emma's lover/boyfriend. Obviously he wasn't a big fan of Emma's ex-boyfriend Will. I do recall that episode where it looked like Emma was going to die after going to Brandon James house, but she ended up in her bed somehow. So this could be a red herring or a possibility. However, I feel like it's odd for him to help her and be the Sherriff's son. I can't really rule him out, because he could've easily called/text Emma to threaten her from being in another room. (Since I think Emma got a call/text from Ghostface sometime when they were together. Which should rule him out, but they weren't together in the room... so, I won't rule him out just yet. Furthermore, it'd be a TWIST if he was the adopted son and had some strange, poetic revenge to seek out Emma and her mother. (Yes, this would be slightly disturbing because, they would've committed incest.)
Kevin Duval/Mystery Child: We don't know enough about either of them. Kevin Duval sounds like he has a lot of loathing towards Maggie's past lover Brandon James and the child she gave up for adoption. So, he could have a motive to create discord between Emma and her mother. As well as try to take Maggie back from Sherriff Clark. However, I feel like we don't know enough to be absolutely certain of it. As for the Mystery Child, they could've tried to seek Emma and Maggie out only to learn of what little Emma knows and attempts to kill her friends/loved ones for having it all? I've already theorized on this some. So, I may change or edit this depending on what we learn in the two remaining episodes.
Brandon James - I know it was said he was shot. However, was there a body? From what I recall, I think it'll be a no. If he's still alive, he could be someone else seeking revenge on Emma and her mother Maggie.
Random Side Note: Emma and her family has always been the target from what we can tell. Why? Because what way to hurt someone more then by hurting whoever is closest to them?
Those I'm not Including: Lorraine, Deputy Mike, Deputy Haverland, Howard Anthony.
^ Lorraine could be bitter for losing Sherriff Clark to Maggie Duval. However, I feel like she's too tall to fit the bill of Ghost Face. Howard Anthony might of wanted revenge for the video that went viral thanks to Nina and Emma. (But I doubt he'd go after everyone or the love interests of his own daughter.) The rest I don't see trying to kill a bunch of kids and haven't had much appearances in the show.
Spoiler