It's important because schools are for education , not indoctrination. Religion is a matter for families and churches etc outside of the school. It has no place in our education system except in religious instruction classes.
one of the worst experiences of my childhood was having to attend compulsory religious assembly. I was born a jew and had been taught a great deal of the history of Christian persecution of the jews-I had even long lost cousins who died in the holocaust. To have to sit through those services was extremely upsetting. My parents eventually asked the headmaster to allow me to miss them, but even then, I was made to feel like a pariah, sitting for 20 minutes on my own in a classroom.
Religion has no place in a school-this is a matter for the home and religious churches, synagogues etc and is a violation of many children's human rights forcing them into situations which can cause discrimination and hurt.