Back in the days of hot metal in old Fleet Street, there were alleged to be a whole series of practices, which were not allowed for in contracts of service. They were known as Spanish practices or Spanish customs, such as false names on worksheets and payments for a day’s work when half an hour had actually been worked.

New heads sometimes find that there are various practices and interpretations of agreements that have crept into use in the school they take over. When they question them, they are told it is custom and practice in this school or local authority.

So what status does custom and practice have? The cynical lawyer’s saying goes: “Custom and practice is the last refuge of a scoundrel when no better arguments are to hand.”

The fundamental basis for the relationship between the employer and employee is the signed contract. For teachers in maintained schools, this will incorporate the statutory School Teachers’ Pay and Conditions Document and agreements explicitly incorporated in it, and usually the Burgundy Book agreement, which defines areas such as resignation dates. Staff covered by transfer of undertakings (Tupe) retain those conditions of service unless they agree to change them or until changes can no longer be considered to be “related to the transfer”.

Custom and practice, rather like a footpath, is established by long and regular use, to the point that it can be assumed to be included in the contract. The rule of thumb is that it must be certain (it always happens), notorious (everyone knows about it), and reasonable. If these do not apply, it is not part of the contract. Moreover, these practices can be changed (although consultation may be needed). A head should consult the school’s HR adviser, but she should not be prevented by a slogan from making a necessary change.

Richard Bird, Legal consultant to the Association of School and College Leaders.