Thanks, that's reasurring.
My son often sees me changing clothes, topless, in underwear, etc. He was breastfed until he was 3.5, has absolutley no idea that breasts could ever be for anything other than feeding babies, animal or human
Hardly ever sees anyone breasfeeding, but when he does, it's perfectly normal to him.
I have watched boys in his class at age 7 giggling at the poster of a woman exposing a bit of cleavage and my son asked me why they were laughing 
I reckon he is just very innocent and young for his age, and that's how I would like to keep it until we as parents are ready to tell him or he asks questions. He does knows he grew up in something called a womb (not in my stomach !), as he used to ask about it often when he was younger, etc.
But he has never asked how the baby gets there in the first place !
He is not ready to be told about fertilisation. I am sure one of his better "informed" mates will tell him soon enough, which is fine. He may at that stage come home and ask us more questions.
I just don't wish the topic to be introduced by other adults when he is blatantly NOT mature enough to deal with the topic.
I agree that many teachers are great (I am a former college lecturer so learned to be very diplomatic with a range of people as I am still involved in adult education) unfortunately my son attends an Ofsted Outstanding school which is far from Outstanding in teacher quality and far from diplomatic.
Once you lose trust in the system, that's it, every little thing becomes a worry.