Bomb disposal experts were called to Buxton Community School yesterday (Tuesday) after concerns were raised over a chemical found at the site.
Derbyshire police said they were called at around 3pm by the fire service.
A controlled explosion was carried out by the bomb disposal team, away from the site, and a cordon was put in place around the school for safety.
Police said there were no suspicious circumstances.
In a statement on the school's website, headteacher Craig Yates said: "On Tuesday 2nd October, our science team identified some potentially dangerous chemicals in our stores, we then implemented a well-established critical incident plan which involves Derbyshire County Council and a range of emergency services. We also took the decision to isolate parts of the building and cancel community activities to allow the emergency services to operate more freely in an empty school.
"We are very grateful to all the emergency services who attended so quickly and offered expert analysis and advice.
"Ultimately, after carefully assessing the situation they decided the quickest, safest and most effective way to remove and dispose of the chemicals was to call the Army Bomb Disposal Unit who “evaporated” the chemicals in a controlled manner on our playing fields at around 6.30pm.
"School is open as normal today Wednesday 3rd October and we would like to apologise for any inconvenience this incident may have caused."
Derbyshire Fire and Rescue Service (DFRS) said they were called to the school at just before 2pm after reports of potentially dangerous chemicals were discovered.
No fire engines or crews attended from DFRS, but three officers did attend to assess the situation and liaised with the police, East Midlands Ambulance Service and the bomb disposal unit.