Dr Nigel Richardson was head of the Perse School in Cambridge 1994-2008.Imagine a party for six-year-old children. Most are from one school and will know each other, but a small number are from somewhere else. They turn up on time, although they are not sure whether the party is really meant for them and whether they should have accepted the invitation.
The host parent enthusiastically announces games which change from one to another at breakneck speed. At first, these "other" children join in. As time passes, they suspect that some of the games are not quite what they seem and start to hold back.
The organiser bolts on a very elaborate overall scoring system to show how each child is doing. But announcing the scores takes up more and more time that might otherwise have been used for games, and the children increasingly want to go home.
Independent schools, which educate around 7 per cent of children in the UK, are in a similar situation after two decades of the national curriculum.
Legally, we did not have to go to Mr Baker's mid-1980s party, but many of us instinctively wanted to, and had high hopes. It was hard to dispute the overall aim - of guaranteeing all children a balanced education to prepare them better for the outside world.
Within my subject - history - I was glad that more chronological schemes of work would ensure children did not study the same eras over and over again. I had learned about the Tudors and Stuarts at five different stages in my school career, while Napoleon had never impinged on my consciousness.
The Baker proposals made independent schools review what they were about. Yet most "shadowed" these developments rather than slavishly followed: we were keen to protect time for subjects such as classics and music.
Two decades on and the party formula has started to look tired. We are deeply concerned at the watering down of modern language requirements after 14, and our science staff tell us that the new GCSE syllabuses in hard sciences are not a good basis for A-level. Meanwhile there are yet more time pressures increased by the arrival of technology, ICT, Spanish, RE and philosophy. Our top mathematicians have always taken the subject a year early, French followed recently, and if new languages such as Mandarin Chinese and Arabic are to have timetable space, other GCSEs taken in Year 10 may well be the order of the day.
Above all, though, has been the introduction of key stage testing and the distorted picture that league tables can create.
Our sector is increasingly concerned too by the rigid application of the scoring systems and the prolonged foot-dragging over recognition of the I-GCSE. A good many independent pre-preps have adopted key stage 1, but now view the new early years proposals as sinisterly over-prescriptive.
So maybe after 20 years, the national curriculum and all its spin-offs really are at a crossroads.
Independent schools value their independence. The architects of the original national curriculum surely underestimated the huge sea change towards central government prescription and control it has come to represent.
It will be ironic if something that many in our sector welcomed 20 years ago turns into a vehicle for even more separation between maintained and independent schools.
1988: Introduction of the national curriculum and GCSEs
1992: School league tables established for GCSEs
1993: The NASUWT wins a High Court battle against Wandsworth council, backing the union's decision to boycott national curriculum assessments of seven- and 11-year-olds because of the workload they created for teachers
1993: Review led by Lord Dearing leads to slimming-down of the curriculum
1995: Key stage tests introduced
1998: Literacy strategy launched
1999: Numeracy strategy launched
2001: First AS-levels taken, under Curriculum 2000 reforms. Introduction of key stage 3 strategy
2002: New style A-levels regraded to make them comparable to previous year's results, sparking complaints from schools and parents
2005: The Government rejects Tomlinson inquiry proposals for an overarching diploma to replace GCSEs and A-levels, and announces plans for its own work-related diploma
2008: Problems with the marking of national tests by the US company ETS casts doubt over reliability of their results
2008: Secondary curriculum and 14 to 19 diplomas introduced
2010: Primary curriculum changes due.