The unexpected discovery was made under King High School in Tampa after a tip-off from a local historian.
According to media reports, Hillsborough County Public Schools conducted an initial scan of the site after Ray Reed - a cemetery researcher - came forward with information about the land's history.
Using ground penetrating radar, officials found evidence of caskets a few feet beneath the school's agricultural program.
It turned out that the school had been built on what was formerly Ridgewood Cemetery - a burial ground that was commonly used by the city's poor between 1942 and 1959.
It is now believed that there could be as many as 250 people buried there.
"[We will be] delivering these findings to the county Medical Examiner and State Archaeologist today - as outlined in Florida law," said a Hillsborough County Public Schools spokesperson.
The discovery follows on from a similar find earlier this year based on Reed's research - that of Zion Cemetery - which was found to contain some 800 graves which now reside under public housing.
The MacDill Airforce Base is now also investigating the possibility of a cemetery on its grounds as well.
It seems that the secrets of Tampa's past can't stay buried forever.