Independent Association of Prep Schools (IAPS)

Independent Association of Prep Schools (IAPS) is a highly reputable membership organization that, in the UK, requires schools to have been inspected by an independent agency (usually ISI, aside from Ofsted reports) as a condition of membership. 

In the case of schools outside of the UK, member schools are required to have been inspected by a legitimate inspectorate or accreditation organisation (ie ISI, CIS etc) or to have been inspected by a trained inspector (ISI-trained or Ofsted-trained).

Although attentive readers will know we are often skeptical of so-called "Ofsted-trained inspectors" (since outside of the UK there is no guarantee the person under contract has any such qualifications) or their reports (which- no matter how expert and well-trained the inspector, other countries are not required by law to keep in the original form, and schools may feel perfectly free to delete unattractive sections or add in glowing bits about themselves), we do believe one can rely on both inspectors and reports in this case (and in the school's less likely inclination to corrupt said reports), because IAPS eyeballs the reports themselves as part of that condition for membership.

If the report does not pass muster or no inspection report exists, IAPS conduct their own thorough Diagnostic Review Visit (largely with ISI-trained inspectors), and that report then goes before the IAPS Membership committee for approval. Therefore schools coming out the other side can be said to have been well and properly inspected. IAPS "is also seeking to agree arrangements for regular re-inspections, so that it can ensure accreditation standards are maintained on an on-going basis". This should ensure  schools keep their socks up to stay in IAPS' good books.