TORONTO -- Ontario's education ministry is recommending school boards freeze hiring ahead of the provincial budget and as the government consults on class sizes and hiring practices.

Deputy minister Nancy Naylor sent boards a memo Thursday noting that the government implemented a hiring freeze in June and that school boards may wish to institute similar measures.

When that public service hiring freeze was instituted, the government said it didn't apply to front-line staff such as police and fire services, and Premier Doug Ford said it also didn't apply to nurses or teachers.

Naylor advised the boards this week to defer filling vacancies for retirements and other leaves for teachers and other staff until the minister gives them an update by March 15.

"I am writing to you today to recommend that school boards exercise prudence in making hiring decisions in light of the upcoming Ontario budget and the recent consultation on class size and hiring practices," she wrote in the memo.

The consultations launched in January contemplate the possibility of removing class size caps for kindergarten and primary grades.

NDP education critic Marit Stiles suggested the promise of an "update" by March 15 is ominous.

"The Conservatives, I believe, have just put families on notice that deep cuts are coming to their children's education," she said.

Education Minister Lisa Thompson said in a statement that her priority is ensuring all students have access to a meaningful education.

"To achieve this, we need to take a hard look at how school boards spend their money and make sure every single dollar invested in our education system is a dollar invested in a student's future," Thompson wrote.

Harvey Bischof, president of the Ontario Secondary School Teachers' Federation, called the news "deeply concerning," saying schools are already "worse than stretched thin."

"There are gaps in the fabric of what we can provide in terms of a lot of the services that kids require," he said.

The Ontario English Catholic Teachers' Association said the memo was alarming.

"The message is clear: the government is asking schools to do more with less," president Liz Stuart said in a statement. "This will have a negative impact on student achievement and the learning environment."

The Ontario Public School Boards' Association would not comment on the memo, but it has recently expressed concern that changes to the province's autism program set for April 1 will mean more students with special needs in schools and so far there has been no additional funding to help them.

Laura Walton with the Canadian Union of Public Employees, which represents 55,000 education workers, said the timing of the memo is "really weird," given that need for more supports -- not less.

"I don't think a hiring freeze is going to be the answer to any of those needs," she said.

School boards typically hear from the government in March about what to expect in the spring budget, Walton said, so the sector will be closely watching the mid-March update from the minister.

"It should give us some indicator of what the world is going to look like as we head into bargaining," she said.

Contracts for teachers and education workers are up in August, at a time when the government is trying to slash a multi-billion-dollar deficit.