This is not necessarily a problem for schools, where the Burgundy Book is embodied in contracts. The book established a notional term ending on April 30, but it safeguards rationality by saying that the period of notice is two months, and that where the summer term begins before May 1, the notice period is taken to terminate on the day before the new employer's term begins. The two months' notice still applies, so the rule of thumb that a resignation should be in before half term may not apply when Easter is eccentric.
The book also says that the head losing a teacher may not hold them back from starting in their new school.
In schools that have adopted six, five or four terms, there may be issues every year. They should have reviewed the contracts given to staff when the new pattern was adopted. Some staff may have moved to a school with Tupe rights (Transfer of Undertakings: Protection of Employment).
It is probably the case that the Burgundy Book's two-month notice period and shifting date provision mean the school has sufficient flexibility to cover changing term patterns as well as a moveable Easter, but do check.
Richard Bird, Legal consultant to the Association of School and College Leaders.