Reforms to the GCSE grading system in England has created "huge uncertainty" for schools, the NASUWT union says.
The union says the new 9-1 GCSE grades will increase the pressure on pupils and narrow the range of educational opportunities for young people.
The new grading system is being phased in from this summer, starting with maths and English, with grades 9-1 replacing grades A*-G.
The government maintains the changes will drive up standards.
Education Secretary Justine Greening says a grade four will be seen as a "standard pass" and a grade five as a "strong pass".
But the NASUWT's annual conference in Manchester heard that the introduction of a new grading system was causing unnecessary confusion, with negative consequences for pupils and teachers.
Paul Daly, a maths teacher at Whitworth Park school in Spennymoor, County Durham, told the BBC pupils would be taking the new maths GCSE imminently, amid confusion about grade boundaries.
"We still haven't got our head around what the grade boundaries are because no-one will provide us with any.
"So we're giving them mock exams and then telling them 'we think that your grade might be a four, or might be a five, or it might be a six, or it could be anything because we don't know because we haven't been told anything about grade boundaries.
"All we know is, compared to last year, the marks are very, very low and very few people are scoring very high marks in the exams."
Mr Daly said students were often anxious and confused, and that low scores like 25% meant "a bit of a counselling session goes on" after mock exams.
"You sit down with the kids and you try to make them feel like actually they're doing well, they're working hard and 'I know that last year you would have got a grade B, you're that kind of student, you would have got a grade B last year.
"'This year I think you're probably going to get a five but I have no clue because I don't have any grade boundaries, I have nothing to judge it off, so as much as I want to make you feel better, I can't give you any guarantees."
Languages teacher Candy Mellor from Marden High School in North Tyneside is preparing for the new GCSEs which will be taken for the first time in her subject next summer.
"I feel very sorry for the Year 10s who have only got two years' preparation for this new exam that we're still learning about.
"We are making up examinations that we think that it's going to be like for our Year 10s to practice - but we just don't know what it looks like.
"But I can start thinking about my Year 7s, so they'll get five years' preparation, whereas the Year 10s have got two where we're still working it out and confused.
Claire Taylor from Woodlawn Special School, in North Tyneside, says her pupils, who have special needs, will not even be able to access the new exams.
"I've got no young person who is able to take the current maths GCSE at present."
This makes them "feel different" from their peers, she says.
"They're already struggling, given the fact that they have a learning disability or a physical disability [...] and they are now standing out even further from the crowd because they can't access the same type of curriculum and the same type of opportunities and qualifications as other young people across the education system."
Chris Keates, general secretary of the NASUWT, said the changes - brought in under Michael Gove when he was education secretary - had been driven by "political imperative", rather than the needs of young people.
"The government has consistently sought to portray GCSEs as 'broken' and 'dumbed down' qualifications in order to push through its vision of an elitist, narrowly focused curriculum and qualifications system which risks failing to meet the needs of the majority of young people.
"The changes to exam grading have created huge uncertainty for pupils, teachers, parents and employers which will be difficult for schools to manage.
"Schools already buckling from excessive workload are now facing even more bureaucratic reform and young people, already experiencing rising rates of anxiety and mental ill health, will face even greater pressure to perform."
A Department for Education spokesman said the new GCSEs would provide "more rigorous content" and the new grading system provided "greater stretch" for the highest performers.
"These changes will help young people to compete with the best in the world and deliver the skills that employers tell us they need.
"Nothing has changed with regard to schools being held to account for the proportion of children achieving a strong pass and we are working with Ofqual to support teachers as we implement the new system."