Some schools set themselves up as Christian schools, and a few make it a requirement to be Christian to join. This means that they can preach in assemblies, sing hymns, whatever, in the school without offending pupils who are of different faiths. This is hardly a bad thing. Islam has its own schools, too. It doesn't force you to be a Christian to join, it simply creates an enviroment where the history of Christianity can be taught, where church is a more integral part, etc. Many people think that children of today are losing moral values, church schools are often seen as a good response to this.
If you want to set up a school you ought to be allowed to choose who you teach, surely?

So no, not really a bad thing at all.

Children born to Christian parents will often be influenced by that in their upbringing, but once you reach a certain age you ought to be able to make your own choices. Some can't, obviously, a yes, these are 'manipulated', but the majority seem perfectly able to decide on their own.
If you're a Christian, teaching your children about your faith is felt to be almost an obligation in many cases. You want them to gain from religion what you have (support from your Church, religious experiences, a meaning to life, whatever people think they get from faith).

To be honest aetheists look far cooler than religious folk nowadays anyway. It's more appealing, it's logic and science and reason, it's a rebellion against the old orders...why am I a Christian again?