The incident happened at The Sele School in Hertford on Monday evening.
An air ambulance and several road ambulances were scrambled to the scene in Welwyn Road shortly after 5.15pm, to reports a 12-year-old boy and a man in his 50s had been struck by lightning.
“The child was taken to Addenbrooke’s Hospital in a critical condition and the man was taken to Princess Alexandra Hospital for further care,” said an East of England Ambulance Service spokesperson on Tuesday.
“We would like to thank our emergency services colleagues and members of the public at the scene who provided first aid and CPR while our crews were on their way,” they added.
In a statement on The Sele School website, headteacher Chris Quach said the lightning strike “affected several people”.
He said the 12-year-old boy who was taken to hospital was from another local secondary school.
”The entire staff of The Sele School, the wider school community and I would like to thank the emergency services, the parents and staff who were attending the match and were so helpful and our first aiders who did an amazing job while waiting for the emergency services to respond,” said Mr Quach.
“Our good wishes and thoughts are with the people who were injured.”
The incident came as thunderstorms and heavy rain swept London and parts of the south-east on Monday evening.