But while the decision to shut infuriated some parents' groups, teaching unions said it was the right decision and blamed Wales's growing school ground compensation culture for the caution shown by local authorities and heads.
Eleri Jones, head of Ysgol Brynhyfryd, Ruthin, said despite gritting, black ice was still a danger to teaching staff and pupils.
"We carried out a risk assessment and decided to close," she said during an interview with BBC Wales.
"We realise it is an inconvenience for parents, but it rained on Sunday and then it froze, leaving black ice.
A car could slide in the drive, and injure a child."
Most of the closures - there were 20 altogether on Tuesday - were in Carmarthenshire, Ceredigion, Conwy, Merthyr Tydfil and Rhondda Cynon Taf.
Closures were due to ice or burst water pipes.
David Evans, secretary of the teaching union NUT Cymru, said the decisions to close were not taken lightly.
"It's a decision justified on health and safety grounds. Schools have to protect themselves against potential injuries - there could always be a danger they can't necessarily foresee with ice on the playground.
"We are aware in this day and age if an accident does occur and it could have been avoided there could be litigation."
Gareth Jones, secretary of heads union ASCL Cymru, said heads had to take a decision based on the health and safety of the children.
"I don't think any head wants to close a school. The vast majority of heads are professional and able to undertake a risk assessment. The threat of litigation has amplified fears on health and safety."
Last month TES Cymru reported that Pounds 750,000 had been paid out in compensation to teachers, pupils and parents for accidents in schools, and predominantly in the playground. Most were for falls.
Slipping on ice was cited in a couple of cases where local authorities paid out.
The details were disclosed following a freedom of information request by Plaid Cymru.
The party say that the rise of no win, no fee solicitors in parts of Wales have gripped schools in a compensation culture, leaving heads to act cautiously.
Meanwhile, the cold snap has caused extra work for primary teachers worried their young pupils are not warm enough during breaktime. Teachers and assistants have been lining up pupils to ensure they have hats, coats and gloves on before they brave the cold, but have been struggling.
Some teachers report struggling putting modern finger gloves on reception pupils, and have appealed to parents to send their children to school in old-style mittens, which are easier to slip on.