If your child attends a church or other religious school, it’s highly unlikely that you will be able to opt out of RE as you have signed up to the school’s ethos and practice, and that will include their religious denomination.
If your child attends a school with no denomination, you should be able to opt out. To do so, you will need to talk to the headteacher. You may be asked to put the request in writing, as it is compulsory for schools to teach RE unless parents formally request otherwise.
Having said this, it might be worth finding out a little more about the RE curriculum in your child’s school and what it involves. It is very different to the RE of my, and possibly your, school days, when fundamentally the only religion taught (and practised) was Christianity.
Today, in state schools and in some religious schools children will learn about the main religions of the world including Christianity, Judaism, Hinduism, Islam, Sikhism and Buddhism through sharing stories featuring important people, visiting places of worship and learning about festivals and the main events in the calendar.
Often much of the RE is taught in assemblies and may be a way of celebrating the school’s multi-cultural community. RE should not be seen as a form of indoctrination, but more as a way of exploring the wider world. But if you still feel that you want to opt out, you are within your rights.