Things to include:

Scales and arpeggios - pick a key for a 10-minute warm-up Include double octaves, double 3rds etc.

Quick study, with 5 minutes' preparation time

Work on the chosen Grade 6 piece(s)

Work on other pieces - either on earlier grades for their own teaching or on a post grade 8 piece for themselves.

Discussion of their pupils' progress

Take a technical point, e.g. posture, pedalling, fingering; Demonstrate it all badly and invite the student to correct you. (Great fun, this one!)

Invite your student to sit in on some of your own lessons as an observer.