fɔːɹ mˈɛni jˈɪɹz, mˈɛmbɚʃˌɪp wʌz ɹᵻstɹˈɪktᵻd tə hˈɛdtiːtʃɚz, bˌʌt dˈɛpjuːɾi hˈɛdtiːtʃɚz wɜːɹ ɐdmˈɪɾᵻd fɹʌm nˈaɪntiːnhˈʌndɹɪd ˈeɪɾi fˈaɪv, ænd ɐsˈɪstənt hˈɛdtiːtʃɚz fɹʌm tˈuː θˈaʊzənd.
For many years, membership was restricted to headteachers, but deputy headteachers were admitted from 1985, and assistant headteachers from 2000.