Builth Wells High School has reopened after the 'stink bomb' that led to its evacuation and then closure last Wednesday.
The school remained closed on Thursday and Friday following the incident which led to a 16-year-old boy being arrested.
Special measures had to be put in place to cater for students taking exams but a message on the school's website from acting head teacher Catrin Taylor over the weekend said the police had now "returned the keys" and cleaners had been in to make sure the school could reopen after the weekend.
Ms Taylor said: I am very pleased to announce that the police have finished their very thorough search of the school and have returned the keys.
"On Monday please could pupils go to the room in which they were being taught at the time of evacuation. The rooms will be opened when a member of staff arrives. Once you have collected your possessions please go to your tutor base.
On behalf of everybody concerned I would like to thank everybody for their support during what has been a challenging time for the school and the community."
The teenager arrested in connection with the release of an unknown substance which closed the school has now been released on bail with conditions, while the police investigation continues.
Dyfed-Powys Police said it has now "released the school back to the head teacher’s care".Powys Commander, Superintendent Jon Cummins, said: “I’m pleased to draw this part of the investigation to a close so parents, children and local communities can get back to normal.“This has caused significant disruption and I’d like to thank the whole community for their support while we safely deal with the incident.Dyfed-Powys Police is treating this as an isolated incident, which is not believed to be terrorist related and continues to co-ordinate the response in consultation with, and with the support of, partners.
The school was closed on Wednesday morning when around 500 pupils were evacuated after staff reported a ground floor corridor was full of smoke and an unknown substance had been released. The school initially said a smoke/sulphur bomb had been detonated.
The force says its investigation is ongoing but the incident is not believed to be terrorist-related. Police had previously said the boy was 17.
A number of pupils and teachers presented various symptoms following exposure to the affected area, and a response involving all the emergency services was swiftly initiated.
Dyfed-Powys Police said after it was called to the school, on College Road, on Wednesday it gave due consideration to ongoing national threat levels, and a high level of emergency services personnel were deployed.
Possible casualties were identified and treated by the Welsh Ambulance Service, with most discharged within two hours post incident with relevant health advice offered. There is no ongoing health risk.
Chief Superintendent Vicki Evans said: “We recognise that there may be concern among residents and visitors, and wish to reassure everyone that the area was immediately contained to minimise any risk effectively. Communities may experience an increased policing presence, which should not cause any alarm – this is merely to reassure and support following this incident.
“The safety of those attending Builth High School and the wider public is our main priority, and as a result we have made the difficult decision that the school will remain closed tomorrow with a view to re-opening on Monday, June 18. The closure also currently affects Builth Wells Leisure Centre.
“We would wish to reassure you that we are aware that the closure of the school is a significant concern for parents, children and local communities; and that we are making every effort to conclude this as soon as possible. Contingencies have been put in place by education partners to ensure minimal disruption to those children sitting examinations."