My sister is in trouble with her high school?

My sister is a minor and her boyfriend was caught with drinking and driving. She was not drinking and the school wants to get involved because they heard a rumor that she was drinking. However, the police tested my sister for drinking and it turned out negative. A parent called the school because her daughter does not like my sister. The school forced her to confess that she was drinking. Could the school be held for slander or be sued for false accusations? The police report does not have my sister in any of the report.