Our student handbook is signed by every student and parent each year and it spells out a number of things like "it's the school's responsibility to Create and monitor safe conditions in the school environment". I also found the following in our elementary school handbook: Students must not Engage in Sexually Inappropriate Behavior, Sexual Assault and Students must protect the safety and rights of others. Students must not sexually attack nor sexually abuse another person. Students must not engage in non-consensual sexual intercourse, sexual contact or indecent exposure with another person.
It may be helpful to find the student handbook for your school and highlight how the school has failed to protect your daughter and live up to the rights and responsibilities listed in the document they created. There should also be specific disciplinary procedures in the handbook. I'm pretty confident that if this happened at our school they would send out schoolwide communication about the incident and share steps they're taking to prevent it from happening again (we've received this type of communication regarding pretty serious issues - the type that could end up on social media or the news - and I would rank this issue in that category). That should be the expectation anyway.
I'm so sorry you're dealing with this. I've had a few encounters with our school and sometimes it feels like gaslighting or circling wagons with the way they respond. I don't think I've ever received a response that was something like "we take this seriously, we've taken steps to address this and we'll work to improve in the future", which is all I've ever hoped for. Instead I get back details about why their actions were appropriate - so basically just a bunch of excuses.