Final answer:
The court case that overturned the legality of school segregation as a constitutional violation was Brown v. Board of Education in 1954, which overturned the earlier Plessy v. Ferguson decision from 1896.
Step-by-step explanation:
The court case that overturned an earlier decision regarding segregation as a violation of the constitution is Brown v. Board of Education. In 1954, the Supreme Court unanimously made a historic ruling that segregation in public schools was unconstitutional, overturning the infamous 1896 Plessy v. Ferguson case which had upheld the doctrine of "separate but equal."
This doctrine was challenged on fundamental constitutional grounds, specifically the Fourteenth Amendment, which guarantees equal protection under the law. Brown v. Board of Education established that even if facilities for blacks and whites were putatively equal, the very act of segregating them was inherently unequal, and therefore a violation of the Equal Protection Clause of the Fourteenth Amendment.
The decision was heavily influenced by the global context of the Cold War, with the State Department siding with the plaintiffs due to the negative international implications of America's racial policies being used in Soviet propaganda. Thurgood Marshall, who would later become a Supreme Court Justice, led the legal strategy with the NAACP to directly challenge educational segregation.