In fact the real code uses session and person tables as you suggest. It is exceedingly unlikely that there will ever be more than two roles. People don't have a permanent role, they are either a teacher or a learner in each session. The SessPeople table is what you suggest as a PersonSessions table.

As an aside, additional roles could easily be added if really needed by re-purposing the IsTeacher column as a RoleId column to be a foreign key for a Role table and setting Learner = 0, Teacher = 1 in the new table.