The teacher asked three teenagers to provide sperm samples (in other words: masturbate) so the class could study sperm under the microscope.
One of the students told his parents about the incident. His shocked parents immediately notified the police.
A spokesman for the police said: "It is a disrespectful and bizarre thing to ask a student, we are all horrified."
The school, located in Campo Grande, Rio de Janeiro, says it's equally appalled by the biology teacher's behaviour.