Mary Bousted, general secretary of the moderate ATL teaching union, told delegates the "fight" against all-out academisation can be won.
She spoke just hours before her union unanimously carried an emergency motion to work with other unions "to defend the public service ethos of the profession" and oppose forced academisation.
During the Easter break, the National Union of Teachers (NUT) voted to ballot for strike action against academisation plans - which could take place in the summer term.
The more moderate ATL will now consider "potential industrial action" if the government "continues to impose academisation on England's schools."
Proposing the motion, former ATL president Mark Baker said the government failed to provide evidence supporting academisation, and added "flawed policies" were generating "embarrassment and ridicule worthy of General Melchett of Blackadder fame".
He said: "We are a profession of lions led by donkeys."
In her speech to delegates at the ATL annual conference, Dr Bousted condemned the government's plans, claiming the move to turn all schools into academies was about "breaking the public service ethos of teachers and school leaders" – rather than education standards.
At the conference in Liverpool this morning, she said: "Never has the time been more right for a coordinated response from all education unions to the attack we are facing.
"Alone we can do something. Together, we can do great things. We must fight together to protect our profession, for the sake of the children and young people whose education depends on us.
"And we will fight, and if we fight together, with parents and councillors, with other unions, with politicians, with governors, with the whole of civil society which opposes the madness of forced academisation, then we will win."
Dr Bousted has criticised the latest government education white paper, calling it "a very strange document".
She said: "Written in breathless prose, it paints a picture which, to me, is unrecognisable. It asks us to believe six impossible things before breakfast, including the big whopper – that the forced academisation of all schools will improve educational standards.
"So, what is forced academisation of all schools really about? We know it’s not about education standards – it’s about running schools as businesses and it’s about breaking the public service ethos of teachers and school leaders."
She also used her speech to criticise the current curriculum and the amount of assessment in the education system.
She said: "The curriculum is insane. It is designed by people who know nothing of how to promote enjoyment of, and development in, writing abilities."
A Department for Education spokesperson responded to the speech, saying it was "a shame" she had "focused on the negatives".
They added: “We are creating a dynamic school-led system in which underperformance can be addressed swiftly and decisively, and where parents can play a more active role in their child’s education.
Under this system, improvement is driven from within and through strong schools spreading their influence to support struggling areas, and driving up standards by working together in multi-academy trusts (MATs) to share resources, staff and expertise."