Work to rule is classed as industrial action....
There is no precise definition of industrial action in employment legislation. Put simply, it is either a strike, 'any concerted stoppage of work' (Trade Union and Labour Relations (Consolidation) Act 1992, section 246) or industrial action other than a strike.
This includes 'working to rule', refusing to do certain duties and not co-operating with the employer: in schools and colleges, this could mean refusing to provide cover, or refusing to attend out-of-hours meetings.
It could also include refusing to teach or supervise a particular pupil or student - this is, in practice, the most common issue leading to industrial action.
To be lawful, industrial action must:
be a dispute between workers and their employer relating to terms and conditions of employment, sharing out work, discipline, or union issues,
not involve 'secondary action' or action taken by the employees of an employer who is not involved in the dispute,
not involve unlawful picketing,
follow a secret ballot, for which there is a legally-required procedure and timescale.