Builth Wells High School has reopened after it was closed for three days due to a "suspected smoke bomb".
Dyfed-Powys Police released the school back to the headteacher's care on Friday evening, June 15.
A 16-year-old boy was arrested on suspicion of administering a noxious substance and was released on bail with conditions.
Police have said there might be more police presence in the town as a result of the incident.
The investigation into what happened on College Road on Wednesday, June 13 is ongoing.
Around 20 people were exposed to an unknown substance which caused breathing difficulties and nausea.
The sports centre was also evacuated and re-opened on Saturday.
Given the ongoing national threat levels, a high presence of emergency services were sent to the affected area and a multi-agency response was set-up.
Dyfed-Powys Police are treating it as an isolated incident, which is not believed to be terrorist related.
Pupils had to sit their GCSE and A Level exams in an alternative location while the school was in lockdown.
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.
"The town may experience an increased policing presence, which should not cause any alarm – this is merely to reassure and support following this incident.”
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