For almost a quarter of pupils in Wales, this week isn't "Back to School", it's "Nol i'r Ysgol".

They are part of a record-breaking cohort of children in Britain who'll report back on their summer holidays not in English, but in Welsh, Irish and Scottish Gaelic.

There is a revolution underway in classrooms across all three devolved nations; most advanced in Wales, but gaining momentum in Northern Ireland and Scotland too.

It began in the 1940s with the first Welsh language schools, where pupils would be taught all of their lessons - maths, science, even French - through their mother tongue. The idea was to protect and promote the national language in everyday life. But now, such is the demand for these places that in some cases it's English language schools which are actually under threat.

The shift is striking. When my generation (the 80s/90s kids) went to school, only about 10% of children in Wales went to Welsh language schools (officially referred to as Welsh medium schools). But in 30 years that number has more than doubled to over 20%. And in the next 30 years, the Welsh Government has plans to double it again, to 40%, as part of its vision to achieve a million Welsh speakers by 2050. Meanwhile, pupils at Irish and Scottish Gaelic schools are also both up by an astonishing 33% in five years.

At Clwb Gofal Tywi play scheme in Carmarthenshire, all but one of the 30 or so children are part of this trend, despite the fact that a good proportion of their parents don't speak Welsh themselves.

For some of the children, using Welsh every day is fundamentally about national identity.

"St. David made it up, so if the language dies, St. David will be sad", was one very endearing reason for preserving it.

But most are switched on to the practical as well as the patriotic benefits of being bilingual. One primary school pupil tells me "there are many more opportunities for jobs", and many of the parents here agree.

In fact, academics argue that economic opportunities play a large part in the growing demand for Welsh, Irish and Scottish Gaelic education. Studies point to the growing number of jobs specifying Welsh, in particular, as a requirement. Research partly credits the rise of nationalism in all three nations, which carved out a secure space for the languages in public life and created exciting opportunities in government and media.

But part of it too, is about the trend towards parental choice in education. And at Ysgol Hamadryad primary in Cardiff, the headteacher says that's the real driver behind her school, which opened this year. It's one of three of the capital's Welsh language schools which are getting brand new buildings to cope with soaring demand.

Rhian Carbis tells me: "Comparing results between schools, Welsh medium schools perform fantastically in comparison to their English medium counterparts. Parents are realising that all the research that has been done in terms of cognitive development shows they are giving their children the very best chance in life."

Ysgol Hamadryad primary headteacher Rhian Carbis. Credit: ITV News

But evidence about the benefits of bilingual education is controversial. Professor Diarmait Mac Giolla Chriost, a language expert at Cardiff University, says: "You can't turn to the research and say 'yes, a bilingual education will give you better academic results'. You can't argue it's detrimental either. But very few people would argue against the fact that education in a child's mother tongue is advantageous."

So where does that leave the growing number of children who attend a Welsh language school, but whose parents don't speak the lingo? In the case of Michaela Beddows, it wasn't a choice.

Her son's local school, Llangennech Primary near Llanelli, will covert from a mixed English and Welsh school to a fully Welsh language school this September. She campaigned against the change, concerned his education will suffer as he doesn't speak Welsh. Although the school will still offer him personalised teaching in English, she says she feels discriminated against.

"I'm livid, I'm disappointed, I'm gutted", she tells me. "There are two official languages in Wales - English and Welsh. I think it's gone a bit bonkers for the Welsh language at the moment."

Parent Michaela Beddows was angered by the decision. Credit: ITV News

Carmarthenshire Council says it's simply trying to provide more opportunities for bilingual education, but in other parts of Wales parents have also complained of less choice, as Welsh medium education expands and English medium education contracts.

It is a remarkable turnaround. For centuries, it was Welsh, Irish and Scottish Gaelic that had to fight for any place at all in schools, their use actively discouraged and even punished. But especially for children in Wales, that struggle is now just another history lesson - and there's a good chance they'll learn it through Welsh.