Age ratings could be imposed on school sex education classes as part of a review into reports of "inappropriate content" taught to children.
The Government has called in a panel of independent experts to advise on new statutory guidance for heads, which is expected by the end of the year.
The probe is expected to look at slapping age ratings onto lessons and ensuring schools have high quality teaching materials.
Rishi Sunak has come under pressure from Tory MPs on the issue, led by backbencher Miriam Cates, who has been campaigning to overhaul Relationships, Sex, Health and Education (RSHE).
She previously told MPs: "Graphic lessons on oral sex, how to choke your partner safely and 72 genders. This is what passes for relationships and sex education in British schools."
But teachers said the claims are "overblown, sweeping and supported by evidence which is flimsy at best".
ASCL General Secretary Geoff Barton said a review had always been planned and it was "not now happening in response to some sort of calamity over the teaching of this important subject".
He said: "It is very disappointing to see Education Secretary Gillian Keegan’s implicit criticism of schools when she says the review will restore the confidence of parents.
“In fact, the vast majority of schools teach this subject cautiously, sensitively and in an age-appropriate manner. Claims made about inappropriate teaching are overblown, sweeping and supported by evidence which is flimsy at best."
Mr Barton accused the Government of failing to support teachers to teach RSHE as it continues to "deliver major policies on the cheap, and then is quick to criticise schools".
Education Secretary Gillian Keegan has written to schools to remind them to publish their existing policies and share them with parents.
She said: "I am deeply concerned about reports of inappropriate lessons being taught in schools.
"This urgent review will get to the heart of how RSHE is currently taught and should be taught in the future. This will leave no room for any disturbing content, restore parents' confidence, and make sure children are even better protected."
Keir Starmer told the Government to "get on with it" and publish the guidance.
Speaking to reporters on a visit to Plymouth, the Labour leader said: "Schools are struggling with this understandably. It's a really difficult position that they're put in.
"Understandably, parents are concerned. They want transparency, they want to know what's going on. As a parent I completely understand that.
"That's why we said to the Government you need to put guidance in place, help schools through this. The Government actually promised that a year ago but hasn't done it.
"The Prime Minister now says 'well, I'll do it sometime later this year'. My message to him would be 'get on with it."
Sex education is compulsory in schools in secondary schools in England, while primary school pupils must be taught "key building blocks of healthy, respectful relationships".
The new guidance is expected to be released in the coming months and will be subject to public consultation.
It is separate to guidance for schools on transgender issues, which the Prime Minister confirmed on Thursday will be published "for the summer term".
