Term-time holiday rules: Breaks 'in rare cases only'
Holidays for pupils in term-time should only be granted in circumstances that are "rare, unavoidable, significant and short", says a head teachers' union.
But more clarification was needed over the current rules, which allow such breaks in England under "exceptional circumstances", the NAHT added.
The union has published its detailed guidance on term-time holiday requests.
The Department for Education said there were "no plans at all to change the policy".
Since September 2013, local authorities have been obliged to fine families who take children out of school for unauthorised absences.
Many parents complain the rules are confusing and inflexible, and encourage travel firms to raise prices during school holidays.
On Friday the Local Government Association urged the new rules to be scrapped, saying these did not recognise the complexities of family life.
The NAHT said most of its members would welcome more detailed guidance.
"There is some debate about what 'exceptional circumstances' mean when deciding whether to grant absence for students during term time," the union's guidance said.
"We believe it is valuable to have some guiding principles to back schools in their decisions and provide consistency."
The NAHT stressed the guidance had no statutory authority and was not imposed on schools. However, it is in line with government policy in emphasising that term-time is for education.
"Children and families have 175 days off school to spend time together, including weekends and school holidays," the union said.
"The default school policy should be that absences will not be granted during term-time and will only be authorised in exceptional circumstances."
The guidance said bereavements, recovering from family crises and important religious observances should usually be considered - but breaks should be only for the ceremony and travel, "not extended leave".
"This is intended for one-off situations, not for regular or recurring events," it added.
The NAHT's general secretary, Russell Hobby, said: "Head teachers already have discretion over the granting of absence during term time.
"They rightly prioritise learning over holidays. Head teachers are able to - and do - authorise absence in exceptional circumstances.
"The fundamental principles for defining 'exceptional' are where requests are rare, significant, unavoidable and short."
The guidance was designed to "help with making individual decisions about granting authorised absence in term-time", he added.
A Department for Education spokesman said the guidance clearly supported the current policy, and term-time holidays "should only be granted in exceptional circumstances".
"There are no plans at all to change the policy and no U-turn.
"Head teachers have always been able to decide what exceptional circumstances are, but if they find that the NAHT's guidance assists them in making this judgement then we welcome that."