Emma Robertson, of Digital Awareness UK, who works with hundreds of schools, told The Sunday Times that children who had grown up in the digital age were not aware that checking phone messages during a meal was rude.
Neither did they realise that halting a face-to-face conversation because a message flashed up on Snapchat was impolite.
"Schools have a role to play to model good behaviour and teach children what is or is not socially appropriate," Robertson was quoted as saying.
Another breach was dumping a girlfriend or boyfriend by instant message, she said.
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"Both these can be highly upsetting, but younger students do not see problems around this kind of behaviour even if their parents would be mortified to learn of it," Robertson said.
Meanwhile, another report in the daily said top private schools are to offer parents lessons on policing children's use of smartphones and tablets after a poll revealed that one in two pupils were worried about not getting enough sleep because of an addiction to technology.
However, more than a third of the children surveyed said their parents were terrible role models and they had asked them to stop constantly checking their own devices. Almost half said the request made no difference.