First, let's distinguish between civil and criminal cases. A civil case asks the jury to award money for the plaintiff, as distinguished from a criminal case, in which the defendant either goes to jail or gets a fine. The statute of limitations for a CIVIL assault or battery case in Missouri is generally two years. (For advice on the statute of limitations on criminal cases, consult a Missouri criminal attorney, but I think it is 1 year for a misdemeanor and 3 years for a felony). Although the statute of limitations in a civil assault or battery case is generally two years, you must then ask when the clock starts ticking. In Missouri, the clock starts when "damages are capable of ascertainment." There was a famous Missouri Supreme Court case in 2002 involving a claim that a priest at Chaminade Preparatory High School molested a boy in the 1970s, and the Court held that decades later, it was still possible to sue. Here is an interesting article on that case, which you can read. http://scholarship.law.missouri.edu/cgi/viewcontent.cgi?article=3730&context=mlr
The next issue is "tolling." Tolling means that under some conditions the clock is "frozen." In Missouri, the clock is frozen while you are a minor. In addition, there can be disability tolling for people with significant disabilities. There is also tolling for "fraudulent concealment," when the defendant conceals from you that you even have a case. The bottom line is that you should have your friend consult an experienced personal injury lawyer to see if your friend can still make a case, despite the passage of time. I am terribly sorry to hear about her treatment at the hands of these horrible people, and I wish your friend the best of luck.
You do not provide much information or even a time frame. Each state has different periods of time in which they permit both civil and criminal proceedings to be brought under these circumstances. Moreover, it is probably too late to do anything against the school except expose their failure publicly, which your friend would not be willing to do. She should report this to the local state's prosecutor's office and also consult with an experienced personal injury attorney ASAP to determine if anything can be done.
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In most states, there is no time deadline for prosecuting these crimes. Also, depending on your friend's age and which state this occurred in, there may still be time to sue for damages. Your friend needs to immediately consult with a personal injury attorney in the city where all of this occurred. Find such an attorney here on Avvo.com under the tab "find a lawyer".