Pupils gathered at the school canteen (right) after being evacuated.
An air ambulance was scrambled to aid paramedics after the incident on Tuesday in a Year 11 class of 24 children. Eight pupils were treated at the scene.
All the casualties, some of whom were experiencing breathing problems, were taken to Ysbyty Gwynedd, Bangor, but were later discharged.
Reports this week suggested that a pupil - believed to be a boy - had found the chemical spray and smuggled it into the classroom.
A Gwynedd council spokesperson confirmed that disciplinary procedures had begun at the school.
Fire service safety manager Terry Williams praised the class teacher for swift action in opening a window and evacuating pupils.
Photograph: Caernarfon Herald.