Police are investigating allegations of a "relationship" between a female teacher and a 16-year-old male pupil at Holyhead High School.
Anglesey Council refused to confirm whether the teacher had been suspended.
A spokesman said the council was aware of the police investigation.
A police spokeswoman said: "North Wales Police are investigating allegations regarding a relationship between a teacher and a pupil at a school on Anglesey."
She added: "As the investigation is currently ongoing it would be inappropriate to comment any further."