The document Inspecting Subjects 11-16: Citizenship with guidance on self-evaluation contains information on what inspectors will look for with regard to school councils.

The following are relevant excerpts, follow the link above, which will download the full document.

Many activities that could contribute to citizenship education take place outside the formal curriculum. One example is the pupils’ school council. However, inspectors must check that there is equal access to the citizenship curriculum. Activities followed only by some pupils cannot meet requirements on citizenship.

Where all pupils are involved in the school council process in a meaningful way, for example through discussion of issues in tutor groups and the election of representatives, this counts as part of the citizenship curriculum. However, a visit by a few individuals to a mayoral reception, in isolation, would not meet requirements.

... development of pupils’ skills of participation and responsible action requires that pupils ‘negotiate, decide and take part responsibly in both school and community-based activities ... It should be emphasised that this aspect of citizenship is also an integral part of the National Curriculum and that a school will fail to meet the requirements of the National Curriculum if not all pupils take part.