The disease has been contracted by 23 pupils at Stathern School in the Vale of Belvoir.
Several other children are also showing the early symptoms of infection.
All the pupils and staff at the school are being given antibiotics and a full immunisation programme is being carried out.
Whooping cough
Caused by the bacterium bordetella pertussis
Spread through breathing in droplets
Illness starts with symptoms similar to the common cold
Antibiotics are used to treat the illness
It can be spread up to five days after the start of treatment
The outbreak first came to light last week and officials from Leicestershire Health Authority were sent in to investigate.
Dr Philip Monk, consultant in communicable disease control, said: "Within the school there are currently a total of 23 confirmed and probable cases of whooping cough.
"In addition a number of children have early symptoms which are suggestive of the disease.
"We visited the school last Thursday to carry out tests and gave all the children and teachers antibiotics to control the spread of the disease.
"In addition, all the children at Stathern Primary School have been given a booster vaccination against whooping cough to further protect them."
Swabs have been taken from everyone at the school and tests carried out.