Welsh-language primary and secondary schools must immerse pupils in the language and limit the amount of English they are allowed to speak if Wales is to become a bilingual nation, according to campaigners.
The comments come in the wake of a row that blew up in Wales after parents complained that their children were being punished for speaking English in a Welsh-language primary in the rural west of the country.
Anonymous claims on a website alleged that children in Ceredigion were being rebuked for speaking English and that pupils as young as 4 were being refused permission to use the toilet because they did not ask in Welsh.
But Rebecca Williams, policy officer at Welsh teaching union UCAC, said that teachers trying to establish a Welsh-language environment were right to put limits on English.
"The difficulty is that English is around us all the time," she said. "The best way to get children speaking Welsh and to become bilingual is to create a Welsh-medium environment and have them immersed in it, so that is what the schools are trying to do.
"It requires a bit of determination to create it. It may be that sometimes it's difficult to find the correct balance between enforcement and what others would see as a necessary step to create the linguistic environment."
Ms Williams said she had not heard of any similar allegations being made against schools or teachers and agreed that the concerns should be investigated.
But she added: "The truth is that more and more parents are choosing to send their children to Welsh-medium schools because they realise that having Welsh is an asset in the workplace and in life in general. People are voting with their feet."
One Welsh-medium campaigner, who did not want to be named, said: "Welsh-medium schools exist because parents want them and they expect them to do their job effectively, which is to produce children who are truly bilingual. Parents would be disappointed if their children weren't being immersed in the language."
The allegations have now been withdrawn from the website and a statement has been put in their place. In it, the parents say they have received some positive feedback but have also been accused of making up their claims. They have been widely criticised for not contacting their children's schools first and for cloaking themselves in anonymity.
"Having drawn attention to the issue we now want to move ahead with our complaints as individuals," the statement adds.
A spokeswoman for the Children's Commissioner for Wales, Keith Towler, said he had written to the group to give them advice about their next steps, but she would not disclose the contents of the letter.
The row comes just weeks after the first meeting of a new body that will advise the Welsh government on its language policy. The eight-member advisory group will hold regular meetings with education minister Leighton Andrews to discuss the progress of the Welsh-medium Education Strategy. Launched in 2010, the strategy aims to develop effective Welsh-medium provision from nursery through to further and higher education.
In one section, the strategy addresses a number of issues similar to those highlighted by the parents in Ceredigion when it sets out how much Welsh and English should be spoken in Welsh-medium schools.
"It is generally accepted that at least around 70 per cent of curricular time should be through the medium of Welsh if learners are to acquire a sufficiently sound command of the language to enable them to use it across a broad range of contexts with confidence and fluency," the strategy states.
One of the strategy's aims is to increase the number of seven-year-old children being taught in Welsh. It wants 25 per cent of Year 2 learners to be assessed in the language by 2015, compared with 21 per cent in 2009.
The number of Welsh-medium primary schools in Wales has grown steadily since devolution.
In the 1999-2000 academic year, there were 444 Welsh-medium schools serving 49,545 pupils. In January 2012, there were 461 serving 62,446 pupils.
Out of Wales's 22 local authorities, Gwynedd has the most Welsh-medium primary schools, with only one out of 102 not classed as Welsh medium. Blaenau Gwent has the least, with only one Welsh-medium primary school out of 27.
In the 2011-12 academic year, 21.7 per cent of pupils in maintained primary schools were taught in classes where Welsh was the sole or main medium of instruction.