Gutterballs

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On January 1, 2008 at the Xcalibur Bowling Centre, a disco-themed bowling alley in Surrey, British Columbia, Egerton, the janitor, has allowed two groups of teenagers to bowl against one another after hours. The "prep", Steve, is still bitter after his best friend Jamie, the "jock", had sex with Lisa, a girl with whom Steve was smitten and even took to their senior prom. After Steve and his friends Joey, Patrick, and A.J. harass the "tranny", Sam, they are chastised by Jamie. This leads to a brawl between Steve's team and Jamie's. Lisa breaks up the fight by dropping a bowling ball on Steve's foot just as Steve is about to kick Jamie again. Egerton, brandishing a shotgun, tells the teenagers to leave and they can continue their tournament the following night.
Lisa, having forgot her purse in the arcade, returns to find Steve and his friends waiting for her. Steve proceeds to violently rape Lisa while the others watch. Egerton is oblivious to Lisa's cries for help, as he is downstairs cleaning up the mess the teenagers made. After he is finished raping Lisa, Steve leaves the room. While Steve is gone, A.J. anally rapes Lisa on the pool table; then it's Joey who abuses her. Patrick, who believed the three were only going to "scare" Lisa, refuses to participate in the rape. Steve returns with a bowling pin and prepares to insert it into Lisa before Patrick intervenes. Steve retorts by giving Patrick the pin and ordering him to do it himself. Patrick refuses to do so at first but complies when Steve threatens to do the same to him. In the end Patrick penetrates Lisa with the bowling pin as she screams in pain. The four leave Lisa on the pool table, naked and barely conscious.
The following night, the two groups arrive at the bowling alley to continue their tournament. Steve is joined by his "girlfriends", Julia and Hannah. Jamie arrives late with Lisa, who barely speaks. As the teams begin their first game of the night, Lisa suddenly gets on an elevator. She is confronted by Patrick, who says he's only at the bowling alley to apologize. Lisa doesn't believe him, and insists that he is only there to make sure that she hasn't contacted the police. During the game, the players notice a mystery player on the scoreboard, BBK, but believe it is just a glitch. Dave, one of Jamie's friends, meets Julia in the bar and pays for two beers she ordered. When he discovers that one of them belongs to Steve, he asks if he can urinate in it. Julia tells him that she doesn't care and that she dislikes Steve too but hangs out with him anyway because he's dating her friend Hannah. Julia follows Dave into the bathroom and the two begin to make out. A mysterious person wearing bowling attire exits one of the stalls while Dave and Julia are in the 69 position and forces Julia's head further onto Dave's penis while forcing Dave's head further between Julia's legs, suffocating them both. After they are dead, two strikes appear next to BBK on the scoreboard in the form of a skull and crossbones.
The game continues and Sam goes to the bathroom so she can "freshen up". The person who murdered Dave and Julia is hiding in one of the stalls when Sam enters the girls' restroom and Sam, frightened by the noise coming from the stall, approaches it and is pulled inside. She tries to bargain with the killer, but the killer murders her by forcing a bowling pin down her throat. The killer lifts Sam's skirt up and, using the switchblade Sam tried to defend herself with, castrates Sam lengthwise. Later, Ben and Cindy leave to have sex in a storage room upstairs, but Ben leaves Cindy alone to buy a condom from the vending machine in the bathroom. He dies when the killer hits him over the head with a bowling pin and gouges his eyes out with a sharpened one. The killer finds Cindy upstairs and strangles her to death with a pair of bowling sneakers. Downstairs, Hannah searches for Julia but dies when the killer crushes her head with two bowling balls. Joey goes back behind the pin setters to fix his team's after noticing they were broken. A.J. notices the strikes appearing next to BBK on the scoreboard and questions Egerton about it, who assures him that it is still a glitch. Egerton sends A.J. to an area currently being remodeled with an "out of order" sign to put on an automatic ball polisher that Joey broke earlier. A.J. turns the ball polisher on and finds it already fixed but gets killed when the killer uses it to wax his face off.
Steve, noticing that something is amiss, leaves to find Joey behind the pin setters. Joey lies dead on the ground, headless. Steve is frightened by the killer, who bludgeons him with a bowling pin before sodomizing him with another sharpened pin. As Steve tries to crawl away, the killer smashes his head in with the bowling pin. Meanwhile, Sarah and Jamie continue their game. Jamie reaches into the ball return slot to find his bowling ball, instead discovering Joey's severed head. The two try desperately to escape the bowling alley before coming face to face with the killer. They run and hide in the basement, where they find their friends' bodies. The killer appears with a shotgun and removes the bowling bag to reveal Egerton's face. Sarah discovers that Jamie knew about the murders of Steve, Joey, and A.J. but not the others. Lisa appears, dressed up as well, revealing that she murdered Steve. Jamie explains that Lisa told her father, who turns out to be Egerton, about the rape and Egerton let Jamie in on the plan. When asked why he killed their friends too, Egerton explains that they didn't intervene when Lisa reentered the bowling alley and are therefore just as guilty. Another person dressed like the killer, as Egerton goes on to explain, Another killer (possibly the real "BBK") enters the basement and removes the bowling bag to reveal Patrick's face. Patrick explains that he confessed his involvement to Egerton and wanted to contact the police, but Egerton let him in on the plan instead and he was the person who murdered Joey and A.J., supposedly to make it up to her. Lisa is furious that her father allowed him to live so Egerton betrays Patrick and slits his throat with the switchblade he got from Sam. Jamie, realizing he's out of control, wrestles Egerton to the ground and, after a scuffle, blows his head clean off with the shotgun. Lisa cradles her father's body before grabbing the switchblade and trying to attack Jamie. Sarah grabs the shotgun and kills Lisa.
With the three "BBK"'s dead, Sarah shoots the locks off of the emergency doors and the two survivors escape the bowling alley. Traumatized and, above all, upset that Jamie did nothing to prevent the murders, Sarah turns the gun on him, pumps it saying: "'cuz we're both fucked". The screen cuts to black as a gunshot is heard, indicating that Sarah murdered Jamie and is on the run.

Question: Where is the person who murdered Dave and Julia hiding?
Answer: in a stall