Authorities in Afghanistan are investigating whether about 90 schoolgirls who fell ill were the victims of deliberate poisoning.
The girls experienced headaches, vomiting and dizziness after strong fumes engulfed the school in Kapisa province, north of Kabul.
The incident is the third of its kind at an Afghan girls' school recently.
Strong fumes were reported on Monday and on 26 April at schools in the nearby town of Charikar.
The police say none of the girls is in danger.
Blood samples have been sent to the American airbase at Bagram.
As yet, officials say there is no clue as to what the fumes may have been, or where they came from.
There has been an increase in reported attacks on schoolgirls in Afghanistan in the past year.
These have mainly been in areas of the east and south of the country, where the Taleban insurgency has been gaining strength.
A group of Afghan schoolgirls had acid thrown in their faces in an incident in Kandahar province last year.
Girls were banned from attending school during the rule of the Taleban, who were overthrown in the American-led invasion of 2001.