Table classes = id_class, class_title, etc.
Table dates = id_date, date, etc.
Table classes_dates = id, fk_class, fk_date

I currently have a pk set in classes_dates--id--but want to know if that's really necessary? It's a dynamic table - every time a class gets updated, I delete every row where fk_class = id_class and insert the new classes and dates. Every time I do this the auto-increment value goes up, seemingly unnecessary.

Will the table survive without a primary key, or should I keep it just becuase?