By Katherine Sellgren
BBC News education reporter, Bournemouth
Exam reforms being introduced next year will cause chaos and lead to a fall in standards, head teachers have warned.
Schools are worred about an overload of exam reforms
The National Association of Head Teachers (NAHT) identified five areas of concern including new diplomas, A-level reforms and curriculum changes.
It predicted "curriculum meltdown" as secondary schools in England struggled with training and timetables.
The Qualifications and Curriculum Authority (QCA) said it was reasonable to expect the changes to go ahead.
By 2008, schools will introduce a revised curriculum at Key Stage 3, offer vocational diplomas, introduce new qualifications in functional English, maths and ICT for 14 to 19-year-olds, implement changes to GCSE English, maths and IT and prepare for A-levels designed to stretch the brightest students.
The NAHT called on the government to review "this helter-skelter timetable" and to allow schools to implement changes at their own pace.
Mike Stewart, head of Westlands School in Torbay, said the changes were "greater than we are in a position to take on board".
"There'll be staff that go off with stress, there'll be staff who reduce the quality of their teaching, the students will lose out as a result of that," he said.
Vince Burke warns of logistical problems with teaching diplomas
"Standards will drop, perceived standards will drop."
Vince Burke, head of John Kitto community college in Plymouth, said the logistical arrangements around the new diplomas in particular - where students will study different components in different locations - would be immense.
"There are transport and funding issues, with different elements delivered in different locations," he said.
"You're going to have this whole lot of administration coming up behind it."
He also warned that having to train staff for the new qualifications would lose them valuable teaching time.
'Eye of the storm'
Eric Fisk, head of Tanfield School, said: "It is no exaggeration to say that we are in the eye of the storm - the relative calm of knowing what we are expected to do and, to a certain extent, how we might do it, but knowing what will happen."
Mr Fisk warned the reforms were being rushed through at "ludicrous speed" and had not been adequately piloted, evaluated or resourced.
"It's an unrealistic timescale," he said.
"It's the cumulative implications of implementing it all in 2008 that create the conditions for the storm that could lead to curriculum meltdown in our schools."
Mick Waters, director of curriculum at the QCA, said 2008 would be a "significant year", with the introduction of diplomas and the secondary curriculum review.
Mr Waters said the Department for Education and Skills (DfES) was making "strong plans" to support the introduction of the curriculum and that the feedback from school was positive.
"No-one underestimates the task ahead, but it's reasonable to think it will take shape for the introduction in 2008."
He said it was always the intention that the introduction of the specialised diplomas would "start slowly and build up".
"I think that serious consideration is being given to the concerns being expressed," he added.