So you have 2 tables, 1 for schools and another for sessions, the sessions table has multiple records for each school ?
If yes then you can get the sessions count, in the secondary model, add a field to the "Fields to retrieve",
And use Model.schoold_id in the "Group" fields box, the names of the fields "id" and "schoold_id" depends on the table's primary and foreign keys
Models : sessions
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