While I'm shocked that you find the only scenario compatible with the girl you hate being a murderer the logical one let's have a brief refresher on what you find logical:
In the late afternoon/early evening of November 1st Amanda and Raffaele are seen in Raff's apartment by an independent witness who Raff agrees to give a ride to later that night. At this point Amanda believes she will be called into work that night at the pub. Soon after this Raffaele launches the movie Amelie on his computer.
Meanwhile, across town somewhat later that evening, Rudy Guede is captured alone by the CCTV camera near the cottage. This appearance, compatible with him, a burglar with a criminal history, scouting a burglary target, is merely a spectacular coincidence or psychic premonition. The students can't know that they will be vising the cottage later in the evening, being currently engaged with other plans, thus Rudy can't know he will be visiting the cottage later in the evening. And yet here he is for no reason at all.
Later Amanda finally gets a text message that she wont be needed at work. Then later still Raff's apartment is visited unexpectedly by the independent witness a second time, who drops by to say the ride is no longer needed.
At this point in time Amanda and Raffaele, who have known each other five days and barely speak the same language, are free for the evening and decide to spontaneously leave their cozy apartment out into the cool November evening on the off chance they happen to run into Meredith, presumably to murder her, although this is not clear. Amanda grabs Raffaele's largest kitchen knife that is inventoried by the meticulous apartment manager and can never leave the apartment.
Along the way they happen by chance to run into Rudy Guede, a virtual stranger neither is on a first name basis with and indeed Raffaele has never met him before. While carrying the deadly weapon with possible nefarious plans they spontaneously decide to invite him along for no reason. He accepts.
Once they arrive at the cottage Meredith has either just beaten them home or she arrives soon after them. At this point the digestive evidence tells us the attack happened nearly instantly. Raffaele used one of his knives to violently subdue Meredith while Rudy held down the witness who was his friends girlfriend for no reason. Amanda then plunged the kitchen knife into Meredith's neck, delivering a fatal blow. By some miracle this giant knife delivered a wound exactly compatible with the smaller penknife that was never found. The small never found penknife is the one that left the bloody imprint on the bed, and was the only knife forensically proven to be at the scene.
At this point Rudy sexually assaulted Meredith to help the two white students out by insuring the DNA of a black criminal would be in the victims genitals. He then walked around in her blood getting footprints and handprints around the body. Luckily for Amanda and Raffaele they didn't leave a trace of themselves in the crime scene.
After they were finished murdering their housemate in their shared house turning it into a crime scene and making Amanda homeless, they knew they needed to stage a break-in. Amanda got a big rock from outside and tossed it into an upper story window above climbable metal bars and inside she tossed some clothes around. By a spectacular, astonishing, amazing stroke of good fortune for her, the stranger burglar accomplice she happened to invite, and who left all the forensic evidence, had just gotten released by the police for an arrest related to a break-in at a law office involving a rock smashed upper story window in which clothes were thrown around the room.
Then the police concluded the break-in was staged and discovered the American student who claimed she was home with her boyfriend all evening was actually texting a black man named Patrick about a secret evening, misinterpreting the English goodbye "cya later." After strong arming a statement corroborating the story they knew to be correct out of her they declared case closed. This was all a mistake, but by another amazing coincidence the girl they arrested for the wrong reasons turned out to be guilty anyway once they had a chance to go back to the crime scene and gather the "rest" of the evidence which included stains that tested negative for blood, and her DNA in her own sink.
This is more or less what you actually believe