Mike Corbett of the NASUWT teachers' union said swearing "has just become accepted" in some schools.
He highlighted cases of kicking and punching, with children as young as five biting and spitting at staff.
Mr Corbett said a colleague in a "very well regarded East Renfrewshire school" and another at Jordanhill, one of the top performing schools in Scotland, had both reported "significant behavioural problems".
Speaking at a fringe event at the Scottish Conservative conference in Aberdeen, he said: "What worries me, and this is only anecdotal evidence so far, and I think this is why we need more proper research, but many of our members are telling us that since kids have come back there's more evidence of that quite serious disruptive and often violent behaviour."
The former English teacher said there had been a lack of research into the issue by the Scottish Government, "as if they don't really want to know it's a problem".
He said the union had been trying to address violent behaviour by a small number of pupils before the pandemic.
Mr Corbett, NASUWT's national official for Scotland, added: "We've had disputes with certain schools and certain local authorities about that because there has been, in our view, too much of a move just to accept any kind of behaviour.
"'All behaviour is communication' is a phrase that is often thrown around, and absolutely it is - if a pupil is misbehaving they are communicating something to you that you need to address, of course.
"But that does not excuse violent behaviour towards other pupils or towards support staff or teachers in the classroom.”
Mr Corbett said children who had been at home during the pandemic are starting primary school "just not ready to have any rules", while in secondary schools there are "groups of kids just feeling it's okay to walk about the classroom and wander around".
He said retention of teachers during the pandemic was generally "quite good".
However, a survey in October found 67 per cent of teachers had considered leaving the profession in the previous 12 months.
Mr Corbett said a lot of teachers are exhausted but want to do the best for their pupils, and he is "really worried" many may decide to quit after the summer holidays.
It came as the Scottish Tories called for Scotland's Curriculum for Excellence (CfE) to be scrapped in favour of a return to a "traditional knowledge-based curriculum".
The party said the current approach is "an anchor that will keep dragging down school standards".
Mr Corbett said the bureaucracy around CfE was "insane" and it had never been put into practice properly.
A Scottish Government spokesman said CfE is “the right approach for Scotland and it is viewed internationally as an inspiring example of curriculum practice”.
He added: “Throughout the pandemic it delivered credible results for our children and young people in the face of exceptional circumstances.”