Im pretty sure the legal standpoint was 4 students per instructor (for the on-site theory portion) and then dropping to 2 students for the on-road part.
I cant find any official sources to back that up though
EDIT, found it 
https://www.gov.uk/motorcycle-approved-t...atb-manual
About half way down. Trainee:instructor ratios :thumbup
I cant find any official sources to back that up though
EDIT, found it https://www.gov.uk/motorcycle-approved-t...atb-manual
About half way down. Trainee:instructor ratios :thumbup