One of the strongest arguments against increasing the number of school terms to four (or, heavens preserve us, five!) is that the last week of any term is always the least productive in the school year, closely followed by the first week of any term. With more, but shorter, holidays, one could end up with a quarter of the teaching year being made up of unproductive weeks.
It is also a myth that teachers would welcome holidays at off-peak times as far as holiday prices are concerned - obviously the timing of the high-price season would follow the school holiday dates. Any system which forced schools to operate during the height of the British summer would lead to problems with absenteeism.
G M D FALCONER 10 Walmer Way Deal, Kent