Final answer:
The "separate but equal" doctrine from the Plessy v. Ferguson decision was the legal basis for school segregation before being overturned by the Brown v. Board of Education ruling in 1954, which determined that separate schools were inherently unequal.
Step-by-step explanation:
The legal basis for school segregation in the United States at the time of Brown v. Board of Education was the "separate but equal" doctrine, which was established by the Plessy v. Ferguson decision in 1896. This doctrine was used to justify Jim Crow laws, which enforced segregation in public life, including schools, and maintained that segregated facilities for Black and White people were constitutional as long as they were supposedly equal. However, the 1954 Brown v. Board of Education ruling overturned this, stating that separate educational facilities are inherently unequal and therefore unconstitutional, marking a significant step forward in the fight for equal education.