She continued: "That means they're more likely not to be going to school on a regular basis, and all the consequences that has for their life chances."
Phillipson said parents had to pull together with schools and government to get their children "off to a good start".
Data showed about 18% of pupils were persistently absent in the 2024-25 school year.
This was down from a peak of 23% in 2021-22, but higher than the pre-Covid levels of about 11%.
Schools have always grappled with attendance issues, but they became much worse after the pandemic in 2020 and schools closed to most pupils during national lockdowns.
Attendance has improved since, but it remains a bigger problem than before Covid.
The Department for Education (DfE) said the data from the first week of the 2024-25 school year showed the start of term was "critical" for tackling persistent absence.
The Conservatives said Labour's Schools Bill had dismantled a system that had "driven up standards for decades".
A head teachers' union said more support was needed "outside of the school gates" to boost attendance.
Karl Stewart, head teacher at Shaftesbury Junior School in Leicester, said his school's attendance rates were higher than average and but there was a "definite dip" in the two years after Covid.
"I get why. Some of that wasn't necessarily parents not wanting to send them in. It was because either they had got Covid or other things, they were saying, 'We'll just keep them off now to be sure'," he said.
The school has incentives like awards and class competitions to keep absence rates down, and Mr Stewart said attendance had more or less returned to pre-Covid levels.
"When we have the children in every day the results are just better," he said.
"If you're here, that gives you more time for your teacher to notice you, for us to see all that good behaviour [and] that really hard work - and that's what we want."
But, like lots of schools, he said some parents still took their children on unauthorised term-time holidays to make the most of cheaper costs.
Others, he said, have taken children for medical treatments overseas to avoid NHS waiting lists.
The DfE said 800 schools were set to be supported by regional school improvement teams - through attendance and behaviour hubs.
These hubs are made up of 90 exemplary schools which will offer support to improve struggling schools through training sessions, events and open days.
It said it had appointed the first 21 schools that will lead the programme.
However, Pepe Di'Iasio, general secretary of the Association of School and College Leaders, said attendance hubs were not a "silver bullet" and a more "strategic approach" was needed.
Additional reporting by Nathan Standley