1. They'd have probably filled in 'C of E' on a form, if required to do so, but that was probably just a matter of convention, rather than actual belief.
2. Absolutely not. I've been an atheist all of my life and I would never have anything to do with Sunday school (or church).
3. Up until the introduction of the National Curriculum (in 1988), 'religious education' was the only subject that was compulsory in schools in England and Wales. (Schools were not legally obliged to teach English, Maths, PE, History or any other subjects. The only thing that they had no choice about was teaching RE). So, yes, we had RE lessons but those in primary school were always given on the basis that "some people believe in this stuff" (rather than promoting religious beliefs as facts) and those in secondary school were generally to do with 'comparative religions' or moral philosophies, rather than promoting any particular beliefs.
Chris