A new rule limiting the amount of time teachers can cover for absent colleagues is leaving support staff "demoralised and discontented", according to a poll of staff.
Teaching union the Association of Teachers and Lecturers (ATL) Cymru found that the "rarely cover" stipulation, which came into force in England and Wales last September, has been warmly welcomed by teachers - but not their assistants.
In a survey, 79 per cent of the union's teacher members said their school had a robust system in place, and 67 per cent said that rarely cover was evenly distributed among staff.
One teacher said: "Rarely cover has had a very positive effect on staff morale", and another stated that they "love it". But this was in stark contrast to the experiences of support staff members, nearly half of whom (46 per cent) said that rarely cover has had an adverse impact on their workload.
More than half (58 per cent) said there was no system of supervision in their school if they were required to deliver lessons and assess pupils, and 73 per cent said they had not been provided with cover supervision training.
One respondent said: "It would be nice to be recognised when we have to cover. Usually the comment is, `oh, will you go in and have them this morningafternoonday'."
Dr Philip Dixon, director of ATL Cymru said: "Although the introduction of rarely cover has had a positive impact for many teachers in Wales, our survey results show the silent, negative impact this policy has had on many support staff.
"Support staff members are often being relied on to cover for absent teachers on a long-term basis with little or no training and no additional monetary benefits. This is totally unacceptable. Evidence from support staff across Wales suggests that they are demoralised and extremely discontented."
Dr Dixon renewed calls for the Assembly government to set up a national pay and conditions structure for support staff across Wales, and urged local authorities to ensure the spread of best practice.
An Assembly government spokeswoman said: "We are working with workforce unions and other partners to develop a national structure which properly recognises school support staff as part of the wider education workforce through appropriate induction, professional standards, performance management and continuing professional development."
Although the introduction of rarely cover has caused fewer problems than were feared at first, some have claimed that Welsh-medium schools are struggling to implement the new rule.
Earlier this year teaching union UCAC said many schools, especially those in non Welsh-speaking areas of the country, were finding it difficult to hire cover staff who were suitably fluent in the language, which was seriously affecting classroom standards.