The poll of people aged 14 to 19 in Scotland by charity Zero Tolerance suggests young people are turning to sources such as the internet for advice.
It found less than one-third - 28% - of heterosexual participants and only 15% of lesbian, gay, bisexual and transgender (LGBT) participants thought the classroom was the most common source of knowledge on the topic.
One in six - 16% - of male respondents believed pornography was the most common way young people now learn about sex and relationships.
The survey - released in full in the coming weeks - comes at a time when the debate over how to teach the "facts of life" in schools has been reignited.
The Scottish Government recently issued new draft guidance on relationships, sexual health and parenthood education (RSHPE) in Scottish schools which brought more than 60 responses from NHS bodies, sexual health campaigners, youth organisations, parents, teachers and churches, the Sunday Herald can reveal.
The sex-health poll by anti-domestic violence charity Zero Tolerance, which believes better relationships prevent such violence, was one of those responses.
The RSHPE advice is being updated for the first time since 2001 to reflect the Curriculum for Excellence which has been introduced in schools, and forthcoming legislation on same-sex marriage.
Over the past decade much has changed: an omission highlighted by the responses is the lack of guidance on the impact of technology, with concerns over issues such as '"sexting" - pupils sharing explicit images.
A teacher at a mixed private school wrote: "We have had numerous instances where we have had to help individuals and discipline individuals from S1 upwards over sending inappropriate photographs taken on their phones."
The availability of phone apps such as Snapchat - where sent images disappear within seconds - meant, she said: "Sexual images are prolific in our very young pupils."
These concerns are shared by Mark Ballard, head of policy at Barnardo's Scotland, who said new technology and social media brought new dangers.
He said: "We've been working positively with the Scottish Government to strengthen the existing guidance, but we believe there is also a need for a more extensive review of RSHPE in schools in Scotland."
Ballard said children could now be targeted by abusers through computers, phones and even gaming devices.
"The exponential change over the last decade means children and young people can access the online world in a huge variety of ways, and form relationships in ways that simply weren't possible in 2001.
"This is why it is essential for any new RSHPE guidelines to address these issues and teach children specifically about online relationships and what is and isn't a healthy and appropriate one."
In Scotland, RSHPE is not a statutory part of the curriculum: responsibility for it lies with councils and schools. Responses to the new guidance highlight concerns over patchy provision.
The charity Children in Scotland said: "RSHPE teaching is inconsistent across Scotland. This is unfair and potentially unsafe in relation to young people's sexual health. Schools that aren't delivering need to be targeted."
The Scottish Youth Parliament also flagged up concerns over inconsistency in quality and content of sex education in Scots schools. It said issues raised by young people include "patronising" sex education, poorly prepared teachers embarrassed about topics, and a lack of information about same-sex relationships and sexual health.
Mhairi McMillan, policy director at LGBT Youth Scotland, said young people often turned to the internet for relevant information. She said: "There is a lot of good stuff [on the internet], but also lots of not-so-good stuff. If nobody is speaking to them in terms of their own relationships, they might end up having sex before they are ready to.
"That is why it is really important people aren't shying away from same-sex relationships when they are talking about sex education in school."
LEGISLATION to allow same-sex marriage was approved in principle at Holyrood in November. The inclusion of a "conscience" clause for pupils and teachers in the revised RSHPE guidance has prompted concern.
This says if a teacher, child or young person is asked to "do something against his or her conscience" they should be able to raise it with the school or council, with alternative arrangements being made if possible.
The Humanist Society Scotland noted this was not present in the 2001 guidance and raised concerns it means teachers would "not be obliged" to explain same-sex marriage law.
Soicety education officer Gary McLelland said: "When you are employed by the state to do a professional job, you do that job with professional integrity and you leave your own personal viewpoints behind …
"I don't understand why it is an issue of conscience for a religious person who is a teacher to be required to teach factually what evidence says, what the law is and what health professionals advise."
Teaching union the NASUWT also refers to the inclusion of this clause stating: "Teachers should just be being asked to be factually accurate so personal views should not come into the matter."
Scotland's largest health board, NHS Greater Glasgow and Clyde, warned that the guidance is "insufficient" to ensure all children and young people get "robust" sex education.
Catriona Renfrew, lead for sexual health at NHS GGC, said: "Our concern is that the guidance hasn't ensured there is a consistent standard of sexual health and parenthood education across all schools.
"We have been very open about our concerns particularly about denominational schools, where in our view there isn't the kind of engagement and development work around the curriculum which we manage to have with non-denominational schools."
She added: "The suggestion that same-sex marriage can be taught in different ways in different schools is seriously problematic. We are certainly keen there is very clear guidance about how that is taught and there isn't a situation that develops where people can opt out of teaching that same-sex marriage is the law in Scotland.
"That is going to be quite a challenging issue, obviously particularly for denominational schools."
The draft guidance notes the right of religious authorities to provide advice for their schools will continue "as at present" - which means they can opt out if they chose.
In its response, the Scottish Catholic Education Service (SCES) flagged concern over guidance which says sexual health services should be provided "where appropriate".
It referred to the "absolute inappropriateness" of Catholic schools being expected to promote contraception or abortion services: "It is impossible for Catholic schools to promote or provide such services as this would be direct contravention of the doctrinal basis of our religious and moral teaching."
A spokesman for the Catholic Church added: "We expect the Scottish Government, in its final guidance, to honour the assurances of Nicola Sturgeon and Mike Russell that it has no intention of requiring any denominational school to act contrary to the guidelines/policy established by the Scottish Catholic Education Service or other religious authorities."
The Church of Scotland did not submit a response to the Government on sex education, but the Reverend Sally Foster-Fulton, convener of the church's church and society council, told the Sunday Herald: "The Church believes that sexual health education should be prepared in partnership with parents, carers and teachers, be appropriate for the age and development of the pupils, and include relationship education."
Others have raised concerns the guidance could contravene United Nations advice on children's rights. Charity HIV Scotland highlighted a statement in the draft RSHPE guidance that authorities must be "sensitive" to requests from a parent to withdraw a child or young person from sexual health education lessons.
HIV Scotland chief executive George Valiotis said: "Ultimately, if you are not giving this [sexual health] information to children, you are putting them at risk.
The Scottish Government aims to publish a final version of the guidance in late March.