An outbreak of sickness at Brentwood School involved 78 pupils and a number of staff.
Health authorities were contacted to assist the school to reduce the risk of further infections.
The school initially said less than one per cent of the 1,100 pupils at the senior school were affected.
Headteacher Ian Davies said today: "I understand there is a small tummy bug going around.
"We have had public health in and they think we are doing all the right things. The school is running normally."
A school spokeswoman, Judith Bailey, said: "Absence levels among staff and pupils at Brentwood School were higher than normal last Friday.
"Nonetheless, the pupil absences represented only a small percentage of the total population and there was no need to bring in supply staff to cover lessons.
"The school was able to provide the full range of Friday lessons and activities. However, some of the staff and students who were absent reported symptoms of sickness, although at the time of writing it is too early to confirm the reason for every absence.
"The pattern of illness suggests that a virus is the most likely source and the school is working with Public Health England and the local Environmental Health Department to reduce the likelihood of further infection amongst staff and pupils."
Health authorities were contacted to assist the school to reduce the risk of further infections.
The school initially said less than one per cent of the 1,100 pupils at the senior school were affected.
Headteacher Ian Davies said today: "I understand there is a small tummy bug going around.
"We have had public health in and they think we are doing all the right things. The school is running normally."
A school spokeswoman, Judith Bailey, said: "Absence levels among staff and pupils at Brentwood School were higher than normal last Friday.
"Nonetheless, the pupil absences represented only a small percentage of the total population and there was no need to bring in supply staff to cover lessons.
"The school was able to provide the full range of Friday lessons and activities. However, some of the staff and students who were absent reported symptoms of sickness, although at the time of writing it is too early to confirm the reason for every absence.
"The pattern of illness suggests that a virus is the most likely source and the school is working with Public Health England and the local Environmental Health Department to reduce the likelihood of further infection amongst staff and pupils."
