Media reports of sexual health classes in schools may sound "shocking and terrible", but research showed sex education needed to start before children were sexually active for it to be effective, she said.
And it needed to "start gently" about age 9, with girls taught about puberty, as they often developed before boys.
In the following two years, both boys and girls should receive more in-depth puberty education, so "it would all be normalised". By age 12, education around experiences, relationships and sexual feelings needed to begin, Ms McConnell said.
"Year 8 is when they start to do their hair and that's when they start thinking about who they like and who they want to be attractive to."
Children in classes she taught told her that casual sex at parties was "the norm" and statistics showed 20% of 13-year-olds in New Zealand had had sex, and 10% had had it in the last three months. Of that 10%, 39.4% had had more than three partners in the last three months.
The rate of sexually transmitted diseases was six times higher than in Australia, and up to 80% would have had sex by the time they left high school, she said.
One in seven young women would have had an abortion by 25, and about 65% of people did not use protection when having casual sex.
The young men she encountered believed condoms were uncomfortable, not very strong, and expensive, and thought "casual sex is where the fun is".
On the other hand, girls were often taught that sex was bad and "pleasure does not come into it".
Adding alcohol to the mix only heightened problems with unprotected and regretted sex.
Ms McConnell believed children needed to be taught about feelings, relationships, communication, and "who they are as sexual beings".
"It's getting the value back in sex.It's really promoting self esteem and self respect. These issues are so complex and unless we help break them down ... nothing is going to change, and that only comes from effective, relevant education."
Parents were often "terrified" about talking to their children about sex, "because they know their own journeys", she said.
This meant teachers were often left to cover the subject, and while there were those who taught it well, some "don't know what they are doing".
"We are still in the Dark Ages with our sexual health."
However, children learnt "so much" from television, movies, and, especially, the internet, and had many questions which needed answering, or ideas which needed debunking.
"We think they are naive and at the 'hand-holding stage', but they are wanting to know about the ECP [emergency contraceptive pill] and STIs [sexually transmitted diseases]. Kids are not at the hand-holding stage.
"We as parents and teachers are at the holding hands stage and we are holding the hands of parents who are terrified.
"I acknowledge that it's scary and in a perfect world we would not have to be teaching 12-year-olds about intimacy and physical contact, but we do.
We are empowering them. Knowledge is power. Our kids have to be trusted."
The one piece of advice she had for parents concerned about what their children were learning when it came to sex, was "talk".
Parents should be giving their children the message that they should have sex with a person they knew, loved and trusted, that it should feel right, physically and emotionally, and that they should protect their body.
"[Otherwise] they have not prepared their child to negotiate and not have sex until they are ready.
"There is this real myth that talking about sex encourages sex. It really doesn't. It promotes sex in a relationship, delay, and contraception use," Ms McConnell said.
She did understand, however, that sexual education needed to be done "respectfully" and that children should not be forced to do anything they did not want to.
One or two parents, out of 30 or 40, would choose to withdraw their child from sex education classes, but the majority were "really pleased" their children were going to be better prepared and educated.
