Pupils are sent home as schools struggle to contain latest outbreak of the vomiting bug.
The biggest outbreak since 2002 of a highly contagious stomach bug is taking its toll on schools in Wales as they return from the Christmas break.
Dozens of vomiting children have been sent home this week and schools are experiencing staffing shortages as teachers also fall sick with the norovirus.
All schools say they have stepped up hygiene procedures to stop the bug - commonly known as the winter vomiting disease - from thriving. Letters have been sent home to parents and special assemblies are planned to warn young children about the importance of hand-washing.
Helene Mansfield, headteacher of Croesyceiliog Comprehensive School in Cwmbran, had 18 pupils and two teachers off with the virus on Tuesday and said numbers were still rising.
"We are keeping a close eye on pupils," she said. "If they display any symptoms we call parents to take them home."
Every school in Wales opened this week on the advice of health officials, despite suggestions they should stay closed for a further week - like some hospital wards - in a bid to avoid an epidemic. Last week eight hospital wards in Wales were shut.
It is estimated that 10,000 people in the UK are currently coming down with the bug every week. Norovirus is said to spread like wildfire in hospitals, schools and care homes and most affects children and the elderly.
But health bosses, who issued guidelines to stop its spread within school gates last week, do not back closure unless absolutely necessary.
Instead, they say that every teacher or pupil with the bug should stay home for at least 48 hours after they recover as they are still infectious.
Caroline Newman, head of 400-pupil Gladstone Primary School in Barry, said it had had an outbreak early last October affecting around 40 pupils and five staff. Letters were sent out to all parents in December, urging them to keep their child home after they felt better.
"Before this we had children coming in and being sick in school. Parents were trying to get their children back as quickly as possible but it was too soon. I will be telling pupils about the norovirus and the importance of hand-washing in a special assembly on Monday."
At St Helen's Roman Catholic Infants in the same town, a young boy had to be sent home on Tuesday after vomiting in the corridor, prompting fears the virus could spread rapidly.
Dr Roland Salmon, communicable disease surveillance centre director at the National Public Health Service for Wales, said children and teachers feeling ill should stay away from school.
The virus, which starts with nausea followed by projectile vomiting and diarrhoea for up to two days, can be passed on by contact with an infected person, contaminated food and water or infected surfaces and objects.
Dr Salmon said there had been unusually high cases of the virus this year.
"The infection, though unpleasant, usually resolves in around 24 to 48 hours," he said.
Things that can be done to avoid spreading the disease include careful hand-washing, not sharing towels, using gloves when handling soiled clothing, using the "hot wash" on the washing machine and using bleach-based disinfectant.
"Outbreaks in semi-enclosed environments, such as schools, can be difficult to control and longer lasting because it is transmitted more easily between person to person and survives easily within the environment," he said.
Ruth Jones, Head of Duffryn Infants in Newport, was among 11 staff affected by the norovirus which forced closure of her school for two days in December 2006.
"Half of the 10 teachers were off and a large number of the 150 pupils. The former head rang the council and it was decided to shut on Friday and the following Monday. The bug is horrendous.
"Afterwards staff became even more vigilant about pupil hand-washing.
"We had an expert in - the children used special soap, then put their hands under an ultraviolet light to see bits they had missed, but we have had no symptoms reported so far this term."
A spokeswoman for Hawthorn Infant School said it had several cases of children and teachers being sent home before Christmas.
A spokesperson for the Health Protection Agency said: "There is no specific treatment apart from letting the illness run its course.
"It is important to drink plenty of fluids to prevent dehydration, especially the very young or elderly."