The Government has proposed the new law as a safeguard against adults preying on vulnerable youngsters if the age of homosexual consent is lowered to 16.
But the National Association of Head Teachers said youngsters would be better protected by professional codes of conduct and strong action by employers rather than laws against behaviour that would inevitably be difficult to define.
The association said that improper relationships between pupils and teachers in schools were already regarded as a "fundamental breach of trust", regardless of ages of consent.
It added that lowering the age of homosexual consent to 16, to match that for heterosexuals, should make "no difference" to the seriousness with which relationships between staff and children were treated.
The association said that trying to define what would constitute "a breach of trust" would be extremely difficult. Too narrow a definition, limiting it to intercourse, buggery and gross indecency, might prevent employers from taking action against behaviour that they considered inappropriate but which did not fall within the scope of the law.
On the other hand, too broad a definition could be Draconian, and risk criminalising consensual behaviour that did not warrant such drastic action. Particular problems could arise if children had been involved in a relationship with an adult before they found themselves in a pupil/teacher situation. A criminal offence would be even more impractical if the two parties were married.
The association said that professional regulations due to come into effect in 2000 would empower teaching councils to take disciplinary action against teachers found guilty of unacceptable professional conduct. David Hart, the general-secretary of the association, said the Government should look to employers rather than the police to ensure proper standards were maintained.
"The NAHT cannot support the creation of a new criminal offence for such conduct," he said. "Instead, a very strong message should go to all employers, if there is equalisation, urging them to do what the vast majority already do: treat abuse of trust as a serious disciplinary matter which will warrant dismissal in the vast majority of cases."
The Government promised, after the House of Lords rejected a House of Commons attempt to lower the homosexual age of consent to 16, that it would examine ways of protecting youngsters from predatory advances from adults.
One of the main complaints from opponents of lowering the age of consent was the risk that the move would leave young people open to abuse from people in positions of trust.
A new attempt to equalise the ages of consent is expected in the autumn.Reuse content