Stuart Robert has said independent schools do not accept “dud teachers”, sending the “bottom 10% of teachers dragging the chain” into the government system, where they are protected from being fired.

The acting education minister made the remarks at an independent schools conference on Thursday, revealing the federal government will aim to lift teacher standards by revamping course content and bringing forward the initial literacy and numeracy test for teachers.

Robert also said the Morrison government was prepared to delay approving the new proposed school curriculum until Australia’s “Christian heritage” was reinserted and “nuts” tasks such as year two students “identifying racist statues” were removed.

Robert noted that Australia’s Programme for International Student Assessment test results have slipped in the past 20 years, from fourth to 16th in reading, eighth to 17th in science and 11th the 29th in maths.

Robert is filling in for Alan Tudge, who opted to stay on the backbench after an investigation found there was insufficient evidence he breached ministerial standards.

Robert accepted the decline had occurred across the board, but also singled-out independent schools for praise, suggesting the government wanted to bottle their success and “take it across the country”.

Although there were no “silver bullets”, Robert said, teacher quality came “screaming out” as the most important fix, as studies showed that bringing the bottom 10% of teachers up to the standard of the average would reverse the decline.

“Now, I don’t think that’s a problem in your schools, because frankly you can hire and fire your own teachers,” he told the Independent Schools Australia and Association of Heads of Independent Schools of Australia national education forum.

Referring to the principal of his alma mater, Rockhampton grammar, Robert said there was “no way … Dr Moulds would accept a dud teacher in his school, like, for a second”.

“So for your school you just don’t have them – you don’t have the bottom 10% of teachers dragging the chain,” he said.

“But for every teacher you don’t have in your organisation, guess where they go?”

Robert said Australia must “face the brutal reality” and “stop pussyfooting around the fact that the problem is the protection of teachers who don’t want to be there, who aren’t up to the right standard”.

Robert said there were teachers who graduated from university who “can’t read and write”, evidenced by the fact they could not pass the Literacy and Numeracy Test for Initial Teacher Education Students (Lantite).

Robert said the government could not accept the current rates of 10% of teachers failing the Lantite in the third and fourth year of their education degree, arguing the test “needs to come forward” to the first week of the first year of the degree.

“If you can’t perform in basic literacy and numeracy, how can you teach children?” he said.

The Australian Education Union president, Correna Haythorpe, said Robert’s comments “are absolutely shameful and can only be viewed as insulting to the dedicated, high quality public school teaching workforce”.

“Public school teachers have always been an easy target for politicians like minister Roberts who think that a cheap and easy headline which attacks teachers about declining educational outcomes will let his government off the hook for their failure to prioritise public education,” she told Guardian Australia.

Haythorpe accused the Morrison government of an “outrageous preference for the private school system”.

The Queensland education minister, Grace Grace, said the comments were “outrageous, inaccurate, and an insult to hard working teachers across Queensland and Australia”.

Grace said Robert had “been acting in the job for five minutes and thinks he knows it all”, suggesting his speech “reeks of a boys’ club … sneering at the state system that educates around 580,000 students in Robert’s home state of Queensland”.

The Victorian education minister, James Merlino, said it was “a disgrace” that Robert had attacked government school teachers and accused the Coalition of “obsessing about culture wars” about the curriculum.

On the proposed new curriculum, Robert said the government “will not approve it” until it is satisfied with changes aimed at decluttering and making it more rigorous.

Robert said the maths curriculum should have fewer instructions to “inquire” into topics and more requirements to “master” topics, such as times tables.

Robert, who is also the employment minister, said he was on a site in Hobart that was able to hire only four apprentices when it wanted eight, because other candidates “didn’t have year 11 mathematics” and you can’t work in the trades “if you can’t measure stuff”.

“So there’s four young Australians who didn’t get a job because no one taught them mathematics in year 11,” he said. “Trust me, they didn’t come from your schools. You and I are on a unity ticket on this.”

Robert called for a “more balanced view of Australian history”, arguing that Australia should be proud of its Indigenous history, western liberal and democratic heritage tradition and recent migration.

“It’s essential that students appreciate each one of them. The idea that a draft was presented to me that ostensibly removed the entire Christian heritage of our nation is unacceptable.”

“I don’t want to teach students in year two to identify racist statues. Are you kidding me? That’s nuts.”

Robert said revisions were “travelling very well” and there would be further talks in April, with the commonwealth and Western Australia yet to agree to the new curriculum.