Local government ombudsman Tony Redmond found that Guru Nanak Sikh Secondary School in Hayes had breached the government's school admissions code in nine ways and the admissions appeal code in two ways.
Breaches included asking applicants for photographs, failing to prioritise looked-after children and asking a child to provide a personal statement.
Redmond investigated the school after five families complained that their children had been refused places from September last year.
Redmond said: "Despite the introduction by Parliament of two statutory codes with mandatory provisions, and an earlier ombudsman report critical of how the school had administered its admissions arrangements and admissions appeals, limited regard appears to have been given by the governors to their statutory duties."
Earlier this week local government ombudsman Anne Seex criticised King David Primary School in Liverpool for failing to respond adequately to a critical report on a school admissions appeal complaint.
