Attacks on head teachers by children as young as four are "very common", according to a regional officer for a head teachers' union.
Annie Hovey from the National Association of Head Teachers in Wales (NAHT) said heads frequently told of being kicked, punched and head-butted.
Ms Hovey confirmed the youngest child involved in a case was just four.
Ms Hovey spoke after an 11-year-old was excluded from a Cardiff school for assaulting the head teacher.
On Friday, South Wales Police said the pupil at St Paul's Church in Wales primary school Grangetown, had admitted attacking Susan Jones.
The year six pupil was arrested and received a "reprimand" from officers before being excluded from the school following the incident last week.
Ms Hovey told BBC Radio Wales the case was not unusual.
She said: "Incidents of this kind do cross my desk regularly, and not just 11-year-olds, but children of a much younger age."
"Frequently heads will tell me, 'I've been kicked frequently by children, I've been head-butted and punched'," she continued.
Ms Hovey said such attacks had a major and long-term affect, with some teachers never recovering.
She said one head teacher had left the profession years after an assault, partly because he had never fully regained confidence.
"Frequently there is loss of confidence, and long-term concern," said Ms Hovey.
"Fear often, because they're never quite sure what's going to happen when the child comes back.
"But major loss of confidence, also very significant distress which takes a long-time to dissipate," she added.