Schools using facial recognition systems should consider less intrusive ways to let pupils pay for meals, the UK's data privacy watchdog has said.
A small number of schools use the technology to allow pupils to make contactless lunch payments.
Nine schools in North Ayrshire launched such facilities this week – the council said it helped reduce Covid risks.
But campaign group Liberty said children should not be used as guinea pigs for the technology.
The Information Commissioner's Office (ICO) said schools should "carefully consider the necessity and proportionality of collecting biometric data before they do so".
It said it would be taking the matter up with North Ayrshire council.
"Organisations should consider using a different approach if the same goal can be achieved in a less intrusive manner," it said in a statement.
Details of the Ayrshire schools' facial recognition systems were first reported by the Financial Times.
A spokesman for North Ayrshire council said: "Our catering system contracts are coming to a natural end and we have the opportunity to install IT infrastructure which makes our service more efficient and enhances the pupil experience using innovative technology."
He added that the council was keen to use contactless options given the ongoing risk of Covid-19.
Covid-19 can occasionally spread via surface contact, though scientific research suggests that airborne transmission via respiratory droplets is more common.
The spokesman added that facial recognition made payments faster and that more than 97% of pupils, parents and carers had given their consent for the system to be used.
Two schools in England also appear to have recently adopted facial recognition technology for lunch payments, the BBC has discovered.
Firms that supply facial recognition systems to schools stress the decrease in time spent waiting in the lunch queue.
One such firm CRB Cunninghams notes on its website that its technology works with face masks and can achieve an average serving time of five seconds per pupil.
The BBC has contacted the firm for comment.
But campaigners argue the technology has no place in education.
"Biometrics should never be used for children in educational settings – no ifs, no buts. It's not necessary. Just ban it," a spokeswoman for Defend Digital Me, a children-focused digital rights group, told the BBC.
"The least invasive option should always be the one used, and whether for buying things in the canteen or borrowing library books, that does not need biometrics."
She added that Defend Digital Me had written to the Children's Commissioner in Scotland over the introduction of facial recognition in the North Ayrshire schools.
Emmanuelle Andrews, Policy and Campaigns Officer at Liberty, said: "Children should be free to go to school without being subject to mass surveillance, and should not be guinea pigs for this discriminatory and oppressive technology".
Professor Fraser Sampson, the Biometrics and Surveillance Camera Commissioner for England and Wales, told the BBC he expected public services contemplating the use of facial recognition to think carefully before "deciding to use a measure as obviously intrusive as facial recognition"
He said using the technology in schools was even more sensitive.
There was, he added, an obvious risks of the "normalisation" of the use of biometric data.
School governance bodies should be able to show what alternative methods had been considered before deciding to implement facial recognition, Professor Sampson said.
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