Teachers have said the DUP’s religious conscience clause Bill would cause classroom discrimination.
Paul Givan’s Stormont private member’s Bill seeks a legal exemption on the grounds of strongly-held beliefs.
He sought to change the equality law following legal action taken against a Christian-owned bakery.
The Equality Commission has brought a civil case against Ashers Baking Company after it refused to bake a cake with a pro-gay marriage slogan.
Chris Keates, NASUWT general secretary, said: “Claims that this Bill is about promoting and protecting religious freedoms are spurious.
“The Bill will allow bigotry and intolerance to flourish and will result in hostility to the lesbian, gay, bisexual and transgender community.
“Our children and young people should be able to grow without the fear of prejudice and discrimination.”
The Bill would create a legal clause that would allow the refusal of goods and services on the grounds of strongly held religious beliefs.
Mr Givan has argued that the law had to be rebalanced and said there should be space for difference.
The Lagan Valley representative explained that the clause would not allow someone to refuse to serve an individual who held contrary beliefs.
Instead, he said, it would only enable them to refuse to carry out a service that they believed contravened their own beliefs, such as the creation of a slogan or message that ran contrary to their religious views.
A motion on the issue is expected to be debated at the annual conference of NASUWT in Belfast.
It will call on the Executive to carry out an equality impact assessment into the implications of the proposed Bill.
It will also call on Education Minister John O’Dowd to ensure his department restates its commitment to the Equality Act (Sexual Orientation) Regulations (Northern Ireland) 2006.
Ashers declined the order for a cake with the image of Sesame Street puppets Bert and Ernie below the motto Support Gay Marriage.
Following its refusal to provide the service, the Equality Commission took on the case on behalf of the customer.