The grade 9-10 students all gave blood samples last month for a pH level experiment using the same finger-pricking instrument.

The teacher involved attempted to sterilise the instrument by wiping it between tests with methylated spirits.

But Director of Public Health Roscoe Taylor said that methylated spirits would not prevent the transmission of blood-borne viruses HIV, hepatitis B or hepatitis C and there was a small risk that these viruses could have been transmitted between students.

He said that the incident should never have happened.

Education Minister Lin Thorp has ordered a report into how the experiment was allowed to take place.

She would not release the name of the school involved and said that it was too early to say if the teacher involved would be sacked.

The ABC last night reported that the experiment took place at Dover District High School.

Ms Thorp was only made aware of the incident on Monday, almost a month after the August 9 science class.

The Health Department was only notified last week.

"The risks of transmitting a blood-borne virus in this particular age group of students is extremely low, but we can't say it's zero," Dr Taylor said.

"Whenever blood is drawn, a freshly sterilised instrument should be used for each individual."

Public Health is arranging for the students to undergo two blood tests, six months apart, to ensure they did not contract anything as a result of the incident.

While the delay in reporting the incident was unlikely to magnify the problem, Dr Taylor said that it was "unacceptable".

"It really should have been the case that the Department of Education recognising this as a more critical incident and actually contacted us for health advice earlier," he said.

Parents and Friends president Jenny Grossmith was shocked.

"It's disappointing, I would have thought the majority of adults would be aware of the risks involved around blood transfer," she said.

The report into the incident is expected within two weeks.