"Twenty-five students have been hospitalised at Farkhar hospital and 15 have been sent to Taloqan [provincial capital] hospital where more facilities are available," Abdul Hakim Aziz, head of Takhar's health department, said on Wednesday.
In total 61 students reported sick on 28 May after the incident in Farkhar District. Some of the sick have been advised to rest at home, the governor of Takhar, Ghawsuddin Abubakar, told IRIN.
All the hospitalised students are in a stable condition.
It was unclear what caused the contamination. "We are looking into all possibilities," Abubakar said.
A spokesman for Afghanistan's Ministry of Education (MoE), Zuhoor Afghan said one of the water-wells might have been poisoned by chemicals leaking from a nearby antiseptic tank.
An MoE official, who declined to be identified, doubted the suggestion by some that Taliban insurgents were involved. The Taliban have torched and attacked many schools in the south and east of Afghanistan and oppose girls' education saying it is against Islamic principles, but it is not clear whether they can be linked to this incident.
"Local warlords and militias have a similar mentality to the Taliban and oppose women's education and work," the official said.
Investigation under way
Local police and the security department have launched an investigation into the incident, the governor of Takhar said.
Provincial officials say samples of contaminated water have been taken by a German-led Provincial Reconstruction Team (PRT), based in neighboring Kunduz Province, for examination.
More than 400 schools were attacked and 27 people killed from early 2004 to the end of 2006 in Afghanistan, UNICEF said.
According to UNICEF, in the past two months 10 school incidents in the country, which include torching, attacks and explosions, have been reported.