Inspectors will arrive at schools without warning under a new inspection regime announced this week.
Chief inspector David Bell said he would reserve the right to send in inspectors without notifying schools where there are serious concerns about them.
The joint announcement by Mr Bell and David Miliband, schools standards minister, is part of a government drive to build a new relationship with schools.
Each school will be allocated an "improvement partner" to act as a critical friend and help raise standards.
Ministers also promised to minimise the amount of paperwork sent to schools and want to replace annual governors' reports with a short private school-style profile setting out exam results alongside a description of the year's work.
The inspection changes follow a consultation launched by the Office for Standards in Education in February. This set out plans for new light-touch inspections from September 2005.
Schools will be visited at least once every three years but inspectors will spend no more than two days on site.
Greater weight will be paid to schools' self-evaluation and Ofsted reports will be much shorter, around six pages compared with up to 50 at present.
The notice given to the majority of schools will be cut from up to 10 weeks at present to between two to five working days.
Schools will no longer be labelled as underachieving or having serious weaknesses. Under the new regime, inspectors will rate schools on a four-point scale ranging from one, representing very good, to an inadequate four.
Those receiving grade four will either be placed in special measures or given notice to improve and reinspected within 12 months.
A member of Her Majesty's Inspectorate, the elite group of inspectors within Ofsted, will be involved in every inspection.
Mr Bell said the move will stop schools preparing for inspections and give inspectors a "warts and all" picture.
"Today's proposals will help reduce the burden on schools whilst ensuring that parents benefit from more frequent information about the quality and standard of education," he said.
But heads are protesting. David Hart, general secretary of the National Association of Head Teachers, said: "This is quite unacceptable. The inspection could take place in the absence of the head who might be away.
There is a dire need to reduce the current notice period but Ofsted's proposals smack of an attempt to catch the school unawares."
Three-quarters of those responding to Ofsted's consultation backed proposals to make inspections shorter and more frequent: this was also backed by most teaching unions.
Plans to reduce the notice given to schools were also backed by 74 per cent of those responding.
But a survey of 619 teachers carried out by the National Union of Teachers suggests the profession is split over the proposals.
Almost half (42 per cent) agreed that there should be "little or no notice" for schools compared to 31 who disagreed. However, 48 per cent were opposed to more frequent inspections.
John Bangs, NUT head of education, said: "The high-stakes nature of inspections has not changed. No matter how much the Government claims to trust schools, these changes do not reflect that trust."
Primary forum, 24 A New Relationship with Schools is at www.dfes.gov.uk
THE NEW INSPECTIONS SYSTEM IN BRIEF
* 6-10 weeks' notice before an inspection
* Large Ofsted teams visit schools for about a week
* Inspections at least every six years
* Extensive use of lesson observation, some use of self-evaluation
* Registered inspectors responsible for some reports, elite HMI for others
* Reports within 40 days of inspection and up to 50 pages
* Schools have to write post inspection action plan
* Categories of schools causing concern: special measures, serious weaknesses, underachieving and inadequate sixth formsNew system
* 2-5 days' notice
* Small teams visit for no more than two days
* Inspections at least every three years
* Self-evaluation at centre of inspection, some lesson observation
* A member of Her Majesty's Inspectorate responsible for all reports
* Draft reports seen by school within a few days. Reports about 6 pages.
* Schools required to amend current development plan to take account of inspection
* Two categories of concern - special measures and improvement notice