By finding against the dad who took his daughter to Disney World, five of Britain's most senior judges have created a powerful legal precedent with implications in schools across the country.
Many families will now face a tricky financial choice - either pay more for holidays or shell out for fines.
The law says parents can be fined if their children fail to attend "regularly". The dad in this case, Jon Platt, had argued that he should not have faced a penalty for taking his daughter away during the term. He said that her attendance rate of above 90% meant that she had attended regularly.
Before this ruling, magistrates and the High Court had both agreed that parents of pupils with good attendance should not be fined. That's now turned on its head - with the prospect of more prosecutions, more fines and more requests for absence turned down.
The Department for Education is expected to update its guidelines based on today's judgement.
Some of the consequences of this ruling may not be intended - Jon Platt believes it will alienate parents so much that some will opt to take children out of state schools.
Some parents argue that "inset days", where schools close for teacher training should also be unlawful as a result of this ruling. They argue that closing schools on polling days should also be halted by this precedent.
The law enacted by Parliament was not clear - today's ruling brings clarity whether you agree with it or not.
Parents, pupils and schools now know that unauthorised absence from school can result in fines even if a child has a good record of attendance.