Plans to move Sats from May to June next year have thrown hundreds of primaries' school trips into limbo and cast doubt over the proposed primary leaving certificate.
Heads are also upset that their pupils will now miss out on a lot of what they believe is the most rewarding part of the year and that they will not be with their pupils when they receive their results.
Shifting national tests for 11-year-olds from the second week in May to "the middle of June" was recommended by the Government's expert group on assessment last week and accepted by ministers who plan to introduce the change from 2010.
They believe it will prevent primary teachers from reducing their emphasis on maths and English in the second half of the summer term, leaving pupils "under prepared and out of practice" for secondary school.
But because the exact new Sats date has yet to be fixed, schools that want to take pupils on residential activity weeks in the summer term and are now being expected to pay deposits, do not know whether to confirm their bookings.
There are also fears that some could miss out altogether because the window between Sats and the end of term, when most schools want to book the trips, will now be so much shorter.
The change threatens another of the expert group's recommendations - the primary graduation certificate designed to recognise Year 6 pupils' achievements across a broad range of areas. The experts wanted it to "highlight the child's strengths before he or she progresses to secondary school".
But, according to the group, the biggest measure of their academic achievements - Sats results - "will not be available before the start of the autumn term".
Janis Burdin, head of Moss Side Primary School in Leyland, Lancashire, marks her pupils' achievements with a special leavers assembly, but she will no longer be able to give out academic awards if the Sats results come out the following term.
She is also unhappy that she will have less time to ask for re-marks and will not be with pupils when their results arrive.
"If someone has not done as well as expected we will reassure them and where they have done very well we will share their elation," she said. "But that will all be lost."
Many primaries take pupils away on activity weeks after the Sats. JCA, a company that organises such trips for hundreds of schools, has already been contacted by several worried about what will happen next year.
Janie Burt, managing directorof JCA, said: "It is not clear what is happening regarding the timetable for Sats and most schools like to book 12-18 months in advance, so this could prevent many having the opportunity to take their Year 6 group away."
An Essex primary head, who wished to remain anonymous, said she was supposed to pay a deposit for a trip this week for the last week in May 2010, but now cannot give an alternative date.
"We need the information now," she said. "I am just one voice among hundreds."
Mrs Burdin said she was "horrified" her Year 6 pupils would now miss out on most of the extra activities usually planned for their last half term.
"What we do after Sats is not a waste of time," she said. "These children have worked their socks off and it is lovely to see them getting all the extra special bits of the curriculum. It raises their creativity and leaves them feeling more positive about themselves and education."
A spokesperson for the Department for Children Schools and Families said the change in Sats dates next year was subject to discussions with Ofqual, the Qualifications and Curriculum Authority and other partners but would be announced as soon as possible.