One - belief is being 'taught' as fact in a setting where the teacher, or any one at the front of the classroom, is almost seen as being all-knowing, almost omnipotent Religion is not a fact and many kids lack the ability to apply critical thinking
Two - after seeing the impact a once off incursion has on kids - like the life education vans, ( maybe only ever 2 hours once in their primary school life), the impact of a regular half hour indoctrination is enormous.
Three - the preaching is not balanced, it is Christian based only.
Four - the children of families who opt out are called 'excluded'. They are expected to sit out of the classroom often in hallways and are only supervised. Some schools allow kids to read or complete Jo,work, but many school are forced to let the kids sit and twiddle their thumbs. They are NOT allowed to do anything that is fun like play sports or a game (that might encourage other kids to ask to opt out) and they are NOT allowed to receive any instruction from teachers (which can be see to be an advantage and encourage parents to opt their child out).
Five - the preacher/teachers are NOT qualified teachers.
Six - children have become distressed that their loved ones are going to hell. Once family at our school has two mummies -recently their child came home and stated he hated his two mummies as the bible said they were going to hell and he would be left without anyone. My own Dd originally attended CRE as I too saw little harm - until she told me I HAD to get married so that god would love me. The CRE teacher denied telling her that, but DD insisted that she had learnt in CRE that you had to get married before you were allowed to have babies.
Seven - I no longer believe in the god of the bible - why would I let someone teach my children a belief system I don't adhere to - that would only cause disharmony on my household.