The incident happened around 8 a.m. in a science classroom at Union High School with 24 students present.
County spokesperson Sebastian D'Elia said the acid spilled is commonly used in vinegar and for printing.
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One student said a teacher spilled a vial and burned his hand, and then a fire alarm sounded.
"This is not an issue where there was something that (the teacher) did inappropriately," superintendent Gregory Tatum said. "From my understanding, a handle...just snapped, and it was an accident that was totally unprovoked."
The victims were hospitalized for evaluation as a precaution. A custodian and 19 other students were evaluated at the scene.
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The county HAZMAT team responded, and the classroom was thoroughly cleaned.
"The response here was quick, efficient," Union Fire Department Chief Thomas Bryne said. "Evacuation of the school as practiced constantly, and the director of the safety for superintendent was followed completely and effectively, and it was orderly."
Students were dismissed at 11:30 a.m.