I think your questions grossly mischaracterise SSA's approach which is far more informed, nuanced and positive about SRE in schools than your questions imply.
But taking the questions in good faith - these are superficial responses to an immensely complex issue.:
Sex ed is teaching about an intimate personal aspect of life in a formal school setting to groups of children. Each age group will already contain children with a massive range of life experiences, abilities, differences etc. So pitching the content / differentiating a lesson is already a challenge. Let alone all the adults being comfortable with the material, ensuring that religious and other sensitivities are acknowledged and that children feel safe when discussing SRE.
We all differ in our levels of emotional resilience and discussing sex, our bodies, changes, how we react sexually can be challenging - for many adults let alone children. This plays out in the classroom as well and is massively influenced by levels of maturity and life experiences. Children should never be compelled to participate in age inappropriate discussions & activities in SRE. It's why the age boundary discussions are so important.
There's some fantastic work and materials around healthy relationships and consent being done both in schools and outside school settings. But teaching isn't as simple as "pouring in the knowledge & information" and out comes the informed child. Children absorb information and knowledge in different ways and at different times. Teachers (and parents) understand this when they realise their words of wisdom have been ignored! Your questions seem to imply that schools can easily teach girls not to involved in a coercive, controlling or abusive relationship? Surely a quick look at the relationships board suggests that this is a massively complex issue that SRE lessons in a school won't "cure"
I think people see schools as a respository for the ills of society. "Just teach them to spot an abuser and say no. Just tell them and they'll understand"
In my experience, what really influences children's learning, social development and behaviour is where school is a safe place where children know that their views are listened to and respected. Where children know how to get help. And I'd see SRE as a small part of that child centred approach.
At what age do children need to know about abusive relationships? How do we empower children to seek help when they're being abused without suggesting to all children that they're surrounded by potential sexual abusers? What are the skills to avoid abusive relationships? If it's that easy to teach then this would have been sorted years ago.
There's no easy answers to any of this. But I am clear that children are safer because of organisations like SSA who have diligently challenged some of the grim materials being produced for schools and are making us all reflect on how we teach SRE safely. We should all be grateful for their efforts.