Rules on bubbles isolating

[calamityjam]

Dd is studying A levels in year 2 at our large local college. I got a call telling me she must self isolate for 14 days because a girl in one of her classes had tested positive. So far no problem. The question is who needs to isolate? Apparently only the students who were sat either side of the student who tested positive in each of their classes have been asked to self isolate. I was under the impression that the whole class for each of the students subjects would need to isolate.
My ds school isolate a whole year group in the same scenario, which has happened last week.
Can anyone let me know if this is common practice or law or recommend?

At least at the school I work at, PHE advise close contacts isolate - defined as within 2m for 15 minutes, or face-to-face conversation. In practise we seem to draw an imaginary 2m circle around pupil's seat in class and send those within the circle home, also their friendship group who they spent break/lunch with as told by the student.