The coaching agreement can be viewed from a macro and a micro level. By macro I mean a large overall partnership, covering at least the whole duration of the coaching partnership; and by micro I mean a partnership for the session, or part of a session.
At the Macro Level
Since the students mature during their time at the High School, it is useful to have a signed coaching agreement on paper, based on the one that your school provides. This agreement should be read and signed by the people concerned, such as administrators, teacher/coach, student and possibly parent(s) /guardian(s).
At the Micro Level
At the start of each session there is an opportunity to define / refine / redefine the coaching agreement
Normally you remind each other of the existence of the coaching agreement that makes the relationship one of coach and coachee, rather than teacher and student. Often all that's required is a phrase like "Is it OK that I coach you for the next 5 minutes (pause)?"; or from the student "Would you coach me on my results?"