Water samples taken at four English Schools Foundation (ESF) schools were found to contain levels of lead slightly above government guidelines, the ESF said.
They said the tests were ordered because of the lead-in-water scare. The scandal started at a public housing estate in July.
The ESF said that one sample of water tested at each of King George V School in Ho Man Tin, Sha Tin College, South Island School in Aberdeen and West Island School in Pok Fu Lam was found to have lead levels "slightly above the guidelines". Tests on some 300 other samples taken from ESF schools were found to be "within the guidelines".
The World Health Organisation guidelines say drinking water should not contain more than 10 micrograms of lead per litre.
The ESF did not state the exact levels of lead found.
The government will start testing water from around 80 primary and secondary schools today. It will take around three months to complete water tests at all schools.
Secretary for Education Eddie Ng Hak-kim said starting from today, results of water tests at schools would be uploaded daily to the Education Bureau website. He said if any samples were found to contain excessive levels of lead, the Department of Health would be informed immediately.