Workmen discovered white and brown asbestos while carrying out roof repairs to Holyrood Secondary in Glasgow's South Side during the summer holidays.
The work was completed on August 8, just five days before pupils returned to Glasgow's biggest high school after the summer break.
When the Evening Times approached Glasgow City Council about the asbestos scare they were quick to reassure parents that it was completely safe.
But parents were angry the council had failed to inform them that the work was being carried out.
One mum said: "I think it's ridiculous that we had to find out through workmen that asbestos had been found in the school.
"The council should have been more upfront about it because now we're all wondering what they've got to hide and it makes us suspicious. If we'd had a letter telling us about it, and reassuring us that the school was safe for our children to return, that would have been a much better way of dealing with this situation."
Two types of asbestos were found in the school - chrysotile (white) and amosite (brown) - during roof repairs and council chiefs insisted Health and Safety Executive inspectors were "fully satisfied" that it had been removed safely.
A council spokeswoman said: "During repairs to the school's roof, asbestos was identified and then removed in accordance with Health and Safety Executive guidance. Air testing has indicated that the asbestos has been successfully removed with work to remove the asbestos completed by August 6.
"Repairs on the roof were then completed in time for the start of the new term."
The council said there was "no need" to inform parents because the work was carried out during the summer holiday.
A spokeswoman added: "There would be no need to inform the parents as all the works were carried out under the HSE guidelines during the summer holidays.
"No young people were in the school during this period and all tests carried out in advance of their return at the new term on Wednesday."
An HSE report on their website revealed that white and brown asbestos pose major health hazards. The report, posted on its website, states: "All asbestos can cause cancer and the vast bulk of scientific evidence in the UK and abroad regards the risk from white asbestos as proven.
"There is evidence that the risks from this substance are less than more potent carcinogens such as blue and brown asbestos, known as crocidolite and amosite.
"But it still carries a risk and for that reason white asbestos will be included in the new rules due to come into force later this year."
A spokesman for the HSE said: "G K Contracts Ltd (Scotland) is licensed contractor for the removal of asbestos and HSE was given notification in June that work was to be carried out at the school prior to the work starting."
Just two weeks before the start of the summer holiday, the Evening Times revealed a series of incidents at the school, including a terrifying attack on a schoolboy outside the gates and security fears over intruders.
The school also enforced a lunchtime "lockdown" amid fears about pupil safety and police were drafted in to guard the Govanhill site following an alleged attack on a schoolboy.