Ronnie Smith, general secretary of the Educational Institute of Scotland, warned that children could become society's "untouchables" because teachers feared prosecution for assault.
The union's intervention came after a depute headteacher of a primary in the east end of Glasgow was cleared this week of assaulting a pupil by dragging him across the floor of the dining hall.
Simon Simpson, aged 42, was alleged to have injured the 11-year-old following an incident in the school last November. However, Mr Simpson was found not guilty at Glasgow sheriff court after it was judged that he had acted for the boy's own safety.
The court heard that the boy had a history of disruption and had been suspended six times from the school - once for punching the depute head.
Mr Simpson, who was suspended from his post until the beginning of this term, said that he was trying to get the "aggressive" pupil out of the canteen and away from others after the boy lost control. Just before the incident, the boy had assaulted an asylum-seeker pupil in the playground.
"It is time for the Scottish Parliament to clarify what a teacher may or may not do in exercising control and discipline over the children in their charge," Mr Smith commented. "As the law stands, and in the current climate when even trivial and groundless complaints often end up in court, the only safe approach is for teachers to avoid even touching a pupil.
"That cannot be in anyone's best interests, least of all the children who have become society's 'untouchables'. Teachers' careers, family life and professional standing are being blighted by the authorities running scared whenever a complaint, however trivial, is raised."
The Scottish Executive dismissed the need for further guidance. A spokeswoman said: "As a general principle, physical contact with pupils should be minimised and it is appropriate that, as children grow older, they naturally need less reassurance and comfort through physical contact.
Staff should never touch a child who has indicated that she or he is uncomfortable with it, unless restraint is necessary to protect the child or others from harm.
"The Safe and Well Guidance, published in 2005, states that the main intention of restraint in any situation is to protect a child from harm, and should only be attempted as a last resort."
A spokeswoman for Glasgow City Council said: "We carried out our own detailed internal review and, following the findings, the employee has been back at the school since the beginning of term. The outcome of the court case vindicates this decision."