By Justin Parkinson
BBC News education reporter, at the NASUWT conference
Parents, not schools, are to blame for creating "foul-mouthed, feckless, potentially violent youths", Conservatives say.
The Tories say they would introduce a Teacher Protection Act
Shadow Education Secretary Tim Collins said some families encouraged "rather than condemned" ill-behaved children.
He promised to introduce contracts guaranteeing good conduct in schools, to be signed by parents.
His party would also look at making the penalty for assaulting teachers the same as for police officers, he said.
Currently, most attacks on school staff are treated as common assaults, which usually result in a caution.
For similar attacks on police, the charge of "aggravated assault" is more usual, and more likely to result in imprisonment.
Earlier, the Conservatives had said they would definitely impose that tougher penalty if they came to power.
But Mr Collins told journalists at the NASUWT teaching union's annual conference in Brighton that a press release on the issue had gone "further than it should have done".
But pupil behaviour remains high on the Conservatives' agenda for education.
If parents refused to sign the good behaviour contracts, the school would be able to deny their child a place, Mr Collins said.
He told teachers: "Zero tolerance is not about words or protecting teachers some of the time.
"It is about making offenders pay and protecting teachers all of the time."
The Conservatives are proposing a Teacher Protection Act, which would also guarantee anonymity for teachers facing abuse allegations.
It would also abolish independent appeals panels, which deal with disputed exclusions.
Mr Collins said: "It's no good blaming schools for deteriorating behaviour among young people when parents all too often set such an appalling example themselves.
"It is wholly wrong, and completely unacceptable, that some misbehaving pupils receive encouragement rather than condemnation from their parents."
He added: "The next time any of us come across foul-mouthed, feckless, potentially violent youths, let's lay the blame where it belongs - not with the schools, not with the teachers, but with the parents."
Liberal Democrat education spokesman Phil Willis also addressed the conference.
He said it would be possible to create several new offences, including "persistent intimidation of staff" which, if proven, would mean "permanent exclusion without redress" for pupils.
Mr Willis added: "If an incident of harassment, verbal or physical abuse warrants prosecution in any other public place, it should warrant prosecution if it occurs in a school."
NASUWT general secretary Chris Keates said more needed to be done to protect teachers from attacks.
The conference heard earlier in the week that some had been punched, slapped and spat at.