There is a legal requirement for all schools to provide Religious Education.
This was made legal in the 1944 education act, which stated that "Religious Instruction should be given in every county school". This act also brought in the right of withdrawal for parents and teachers (from teaching it). However, there are specific grounds under which this can be done.
There are guidelines as to how much should be taught, but as far as I am aware, these are guidelines. It does not have to be a set hour a week say, schools can put them all together and do 'RE' days for example, and still fulfill their legal obligation.
There is also an obligation to provide RS in some form for A level students too. Again, this does not have to be a weekly provision.
However, this does not extend to colleges. I teach at a FE college, and we have no requirement to teach any RS.
A few years ago, Ofsted stopped reporting on the RS provision in schools. Since then, there has been a big increase in the number of schools who do not fulfill their legal obligation. with it not being reported on any more, there is little recourse for those schools who do not do this. I'm a HoD in RE (but at a college). I understand from my RE teacher friends in schools that the pressure has really been put on, and schools are cutting back on the amount they teach (particularly with the EBACC changes).
It is a shame.