An outbreak of a flu-like virus has seen about 60 pupils at a junior school struck down with illness.
Public Health England (PHE) confirmed it is investigating the outbreak affecting one-third of pupils at Backwell Junior School near Bristol.
It said testing is under way to confirm if the illness affecting the children is flu.
Parent Jenny Holmes said the school has been "really proactive about telling us about it and keeping us all informed".
Mrs Holmes, whose son Flynn has been affected, said: "Loads and loads of the kids are off school with a combination of with flu and colds.
"My WhatsApp groups are full of people telling me their children are off sick.
"There has been a few times when vomiting bugs have gone round, but never this many have gone down in one go."
In a statement, PHE said: "North Somerset Council and NHS partners are working with Backwell Junior School in north Somerset following around 60 reported cases of influenza-like-illness in pupils at the school.
"Parents, carers and staff at the school have been reminded by Public Health England about the importance of good respiratory and hand hygiene, the protection offered by the seasonal flu vaccine for those at higher risk of flu, and the need to keep children off school who have symptoms."
Nick Webster, headteacher at Backwell Junior School said: "Unfortunately, we have a number of pupils currently off school with flu-like symptoms. We hope that they are feeling better soon.
"Since the illness was first reported, the school's vaccination session has been bought forward to protect the school population and a deep clean of the school has also taken place.
"School nurses have also been available to give advice and information."