Fire Service assistant area commander Nigel Hall told Hawke's Bay Today it was believed the spill occured on Friday, with the mercury leaking into carpets at the school over the weekend.
A Hawke's Bay District Health Board spokeswoman said nine students and two teachers had been brought to the hospital, but it was "very much precautionary".
Earlier today a Hastings fire spokesman said students had been evacuated from one classroom, but was unsure if the rest of the school had been evacuated.
Around 11am the school posted on their Facebook page advising parents of the situation, and asking them to collect their children.
"We have called the Fire Brigade as we have a toxic smell in the Senior block this morning. As a precautionary measure so the Chemical team can do their job we would like you to come down and pick up your children please."