A 16-year-old boy was arrested on suspicion of possession with intent to supply class A drugs, after an incident at Tring School on Friday.
The boy, who cannot be named for legal reasons, was released on police bail while enquiries continue.
A Tring School spokesman confirmed there was incident where a "small number of students took a prohibited substance".
"As soon as they became aware of the issue the school Leadership Team called for an ambulance as a precautionary measure as well as police support," they told the Gazette.
"Any consequences for the students will be as set out in the relevant school’s/trust policy.
"Should there be any further action initiated by police then that is outside of the school’s knowledge or control. All of the students involved are safe."