The victims, ages 14 to 30, went to the hospital in Afghanistan's northeastern Takhar province after suffering from loss of consciousness, dizziness and vomiting, CNN reported. There were no reported deaths as a result of the incident.
"Looking at the health condition of these girls, I can definitely say that their water was contaminated by some sort of poison," provincial health department Director Hafizullah Safi said. "But we don't know yet what was the water exactly contaminated with."
Taliban rule prevented many Afghan women from getting an education from 1996 to 2001, but all-girl's schools reopened after a U.S.-led invasion toppled the regime, CNN said.
Some traditional and conservative families still prevent women from the classroom and female education facilities have come under attack in recent years. More than 100 schoolgirls and teachers became sick in 2010 in a series of poisonings, CNN reported.
Afghan police said they were searching for the perpetrators of the incident at the Rostaq district school, and believe extremists are to blame.
"It is the work of those who are against girls' education and peace and stability in Afghanistan," district Administrator Malem Hussain said.