I think it's called education Maeght. Every society indoctrinates it's youth with the values of that society. A secular school cannot help itself indoctrinating humanist values. Sometimes it produces good people, sometimes it doesn't. The trouble with Humanism is that it looks at people with rose tinted spectacles. I remember seeing Copson expound his solution to crime which was to sit down with perpetrators and talk it through. Teaching that I think could be quite harmful.
So you agree it is
indoctrination, great, because it is. Adults imposing their beliefs on children. You can bring children up and educate them in the culture of the society they live in but religious belief should be a personal thing and shouldn't be imposed under the guise of culture and values.
Historically those of a religious faith have tried to impose their beliefs on others I'm sure you'd agree, hence my comment that people shouldn't try to impose their beliefs on others. It's happened in the past and some try to do it now.
Hope that explains my comment.