You already answered your own question--she filed a police report, there were no witnesses to corroborate your sister's story, so the girl was presumably not arrested, and the school believed the other girl. Without evidence, there is nothing more to do. The school presumably enforced its rules against such behavior, and it is unfortunate that for whatever reason this girl was more believable to them. You use the term plural for "roommates" so I surmise that it was your sister's word against more than one person so whomever was there besides the other girl backed the other girl up. In other words, she had witnesses on her side and your sister did not. The cops are not going to take action without evidence. If your sister has personal injury, she may want to consider suing this girl in small claims court--based on what you state, it is unlikely she has the level of injury that necessitates getting an attorney involved. If she wants the school to reconsider its punishment, she can consult an Administrative Law attorney. But you should know what matters in these circumstances is whether the school followed its own procedures in investigating and dismissing.
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