Emergency crews respond to a chemical explosion in a Grade 11 class at Brennan High School in Windsor, Ont. ((CBC News))

A teacher and three Grade 11 girls suffered minor injuries in a chemical explosion in a science lab at Brennan Catholic High School in Windsor, Ont., on Tuesday morning.

"I understand two were cut more severely than the other students, but all of them are being checked out for ringing ears and possible lacerations," said communications co-ordinator Jill Braido.

Braido said the explosion, which caused students' ears to ring, was an exothermic reaction between the ingredients potassium chlorate, manganese dioxide, sugar and glycerin.

"We're just thankful that it didn't turn out worse than it certainly could have."

The school was in a medical lockdown for about about an hour and a half as Raymo Road was blocked off, and ambulances, fire trucks and a hazardous materials response team were called to the scene.

No exposure concerns

According to Windsor Fire and Rescue, at around 10 a.m. ET, an experiment in a test tube went wrong, causing shards of glass to injure chemistry teacher Steve Pellarin.

"He had his safety glasses on so he did all the safety precautions that were necessary for this experiment. He even had the students move back," said acting district fire Chief Don Bennett. "There's no real concerns of [chemical] exposure at this time."

Pellarin received lacerations on his face and arms, and was taken to hospital in an ambulance.

Chemistry teacher Steve Pellarin was injured during a test-tube experiment that went wrong. ((CBC News))

Chelsey Gascon, 15, said she received a text message from students in the chemistry class. She said students were stunned by what they saw happen to their teacher.

Gascon said students enjoy Pellarin's class.

"He's like really fun. Really funny. He makes science so exciting."

The three female students were also transported to hospital by ambulance. Fourteen other students in the class were taken to hospital in a schoolbus for observation.

Everyone, including Pellarin, was later released. Some required small bandages.

According to the school board, some students returned to school for afternoon classes. Counsellors were also made available to students.

Brennan's principal says there will be an investigation.