The school inspection system is in danger of becoming "a nitpicker's paradise", the headmaster of Eton warned yesterday.

In a speech to the Boarding Schools' Association conference in Torquay, Tony Little said teachers have become "averse to risk" as they scramble to ensure that they tick all the boxes on inspectors' checklists, adding that the "age of measurement" came "at a cost".

He said teachers were becoming increasingly unwilling to take pupils on school trips in their own time, because they know "they are judged most tellingly on the grades their students achieve in public exams". This attitude then rubs off on pupils, "who have somehow picked up that there is an overriding functionality about their education".

Inspectors visiting boarding schools should spend time seeing how students worked in the evening rather then "being in conclave in a hotel", he suggested.

Mr Little's comments come in the wake of warnings from state headteachers who are considering boycotting inspections because, they claim, their verdicts are inaccurate and increase the stress on teachers.

A second teachers' union – the National Association of Schoolmasters Union of Women Teachers – warned yesterday it would not administer national curriculum tests next week in cases where heads were boycotting them.