The null curriculum

The null curriculum is that portion of the curriculum that should be taught but, in fact, isn't. It may result from time constraints, teacher preferences, unavailable resources, etc. It may be an unintended outcome, or it may happen by design.

Other terms that are used interchangeably with the above include 'planned curriculum', 'written curriculum', 'recommended curriculum', 'operational curriculum', 'actual curriculum', 'unplanned curriculum' and 'observed curriculum'.

What is important from all this is that:

we must not lose sight of the fact that curriculum studies must ultimately be concerned with the relationship between intention and reality and, indeed, with closing the gap between them.

Kelly (2009 pg.12)