Teaching unions are concerned that school closures caused by last winter's snowy and icy weather may be used to justify the scrapping of two training days next year.
Welsh schools closed for an average of four days during the extreme conditions in December 2009 and January 2010, according to statistics released by the Assembly government last week.
Although there were wide variations in the number of closures across the country, in seven out of the 22 local authorities every school was shut at some point.
Education minister Leighton Andrews is considering scrapping two of the seven Inset teacher training days from the 201011 academic calendar and said he is looking at how the remaining days could be used to "mitigate the impact of disruption to education that bad weather can cause".
He said: "Although this kind of weather is relatively rare in Wales, parents and I are concerned about the numbers of days of schooling children and young people lost.
"It is important that our schools are open for the business of teaching and learning."
But teaching unions said heads closed schools as a last resort and should not lose valuable training days as a result.
Iwan Guy, acting head of NAHT Cymru, said: "Inset and snow should be entirely separate issues. Inset days are valuable to schools especially now CPD (continuing professional development) funding has been drastically cut."
Gareth Jones, secretary of the Association of School and College Leaders Cymru, said: "Yes, these snow days are disruptive, but you shouldn't penalise all schools for the actions of the minority.
"Heads have concerns about CPD and preparing their staff for the curriculum changes coming forward."
Philip Dixon, director of ATL Cymru, said: "I think we need to be far more level-headed about this. Linking Inset days with snow is confusing - the minister must decide how they can be best used."
The closure statistics for 20 local authorities show a wide variation between different parts of Wales.
Only 22 per cent of schools in Pembrokeshire were closed, compared with 90 to 99 per cent in The Vale of Glamorgan, Rhondda Cynon Taff, Merthyr Tydfil, Torfaen, Newport and Cardiff.
Overall, more than one in five primary schools (220) closed for six days or more, while one in ten (113) closed for seven days or more and one in 25 (47) for eight days or more.
A fifth of secondary schools closed for six days or more and one in 20 for eight days or more.
A total of 95 schools did not fully close - 81 primaries and 11 secondaries.
Headteachers defended their decision to close in the face of criticism from the minister.
Nigel Stacey, head of Dwr-y-Felin Comprehensive in Neath, said: "We understand Mr Andrews wants to respond to parents' concerns, quite a bit of valuable learning time has been missed.
"But it was a very difficult situation for headteachers, it caused us a great deal of anxiety and we would not have closed unless there had been a real health and safety risk. There are always going to be certain schools above the snow line which can't open. Whenever it snows, I can't open my lower school site because you can't grit it.
"Children have come in during our Inset days to catch up on missed work."
Carole Pugh, head of Pandy Infant School in Bridgend, said: "We missed two Wednesdays, but other schools were closed for much longer. It's harder being an infant school because we have to be more careful about children's safety when they are outside."
- Original headline: Cold snap no excuse to put training on ice union leaders insist