An ambulance attended All Saints Primary School, Norreys Road, at around 10am this morning (Tuesday).
All four children and one adult - possibly a member of staff - were taken via ambulance to the Royal Berkshire Hospital, and three children have since been discharged.
A ambulance spokeswoman confirmed they had ‘non life threatening injuries’ - possibly caused by chlorine - and were taken there as a precaution.
All Saints Primary School acting headteacher, Jacqui Kearney, said: “This morning was the first time we have used the pool this summer, and unfortunately four children reacted to chemicals in the water.
“We acted promptly to ensure their safety and well-being. Four children were taken to hospital as a precautionary measure and three of them have since been discharged. All other children appear to be unaffected.
“The swimming pool is now closed and it will be inspected tomorrow to establish what caused today’s issue.
“We are writing to parents today to inform them and remind them, as always, the safety and well-being of all the children at our school is our highest priority.”
A rapid response vehicle, an ambulance officer, an ambulance crew and an emergency care practitioner attended the scene, and assessed a number of other people who did not require treatment.