English-speaking countries are "squandering" the benefits of having bilingual children in their schools and risk turning them into monoglots, according to an international languages expert.
Professor Joseph Lo Bianco, of the University of Melbourne, said school leaders must think of ways to help children maintain their mother tongues, at the same time as teaching them English and "prestige" languages such as French or Japanese.
The professor, who advised the Australian government on its languages policy, said it was of "critical importance" that education systems across the world fostered a "multilingual mentality", with bilingual children being given a strong role to play.
His comments came in the same week that the British Council released a major report showing that three-quarters of UK adults are unable to hold a conversation in any of the 10 most important languages for Britain's future prosperity, including Spanish, Arabic, French, Mandarin and German.
The report calls on policymakers to introduce a broader range of languages to every child's education, and to give languages the same prominence as science and maths. More schools should draw on the language skills of native speakers and cultural organisations in their local communities, it says.
Professor Lo Bianco told teachers at a conference organised by the British Council in London this week: "Schools should not be about making children forget what they already know.
"Hundreds and hundreds of children in your schools and in schools in Australia and Canada and other places begin schooling as potential bilinguals. They have the gift of bilingualism already in their homes and yet we subject them to an education curriculum that assumes that English is normative knowledge and what they already have is some kind of subtractable skill, so we turn them into monolinguals.
"As headteachers and leaders you need to devise ways where, even if we can't teach the languages the children speak in their homes, we can help them reinforce that knowledge, (so) that they maintain it."
Professor Lo Bianco added: "This gift, this donation that they've made to the public education system, is something that we squander. We need to foster it much more."
He also stressed that anglophone countries had a "seriously anaemic" performance in second language education, and that native English speakers must not be fooled by the "false comfort" of their "superficial advantage". The skills gained through learning languages were as vital as the languages themselves, he argued.
The number of bilingual children in schools is rising around the world, including in the UK and the US. Census results show that one in six primary school students in England does not speak English as their first language, and in secondary schools the figure stands at one in eight. In the US, around one in five school-age children speaks a language other than English at home.
Academics have suggested that there are as many bilingual children in the world as there are monolinguals.
Amy Thompson, the recently retired chair of the UK's National Association for Language Development in the Curriculum, which promotes the learning of English as an additional language, said that negative attitudes to immigration had led to support for children's mother tongues being sidelined by some governments.
"These children (and the languages they have) are a great resource to the country," she told TES. "But it can be the case that you enter the school as a bilingual and you leave monolingual.
"Policies just don't support these children. This is an issue strongly linked to politics and the negative associations there can be about immigration - there is this mistaken idea that if you support the mother tongue you are not allowing children to integrate."
Despite this, she praised work in Australia to create state-wide structures to support the bilingualism of aboriginal and immigrant children in schools.
Ian Bauckham, president of the UK's Association of School and College Leaders, said schools were sometimes "stumped" by children who were already fluent in a foreign language. The students were often diverted into learning further foreign languages, which was useful, but this failed to support their existing skills, he said.
"Schools have to take quite imaginative approaches, perhaps by using the child to support the learning of others or assigning them someone specific to support them," he said. "Very often the focus is all on GCSE and accreditation, but that's quite a limited view of what you can do with a bilingual child."
Top 10 languages for Britain's future prosperity and global standing (based on cultural, economic, geopolitical and educational factors)
Source: Languages for the Future: which languages the UK needs most and why (2013), British Council.