Amid a $6 billion building boom to tackle population growth, dozens of schools sit half-empty with space for thousands more students.

These once-bursting schools are struggling to attract enrolments as parents embrace "school shopping" and choose to send their kids to private schools and public schools outside their catchment.

Recent Department of Education figures show there are 43 Sydney schools with a utilisation rate of 55 per cent or less, including more than a dozen high schools. A rate of between about 75 and 100 per cent illustrates an efficient use of space.

Another 52 schools operate at between 56 and 65 per cent.

Secondary schools are a particular problem in gentrifying suburbs, where studies have shown that middle-class families worry about sending their children to schools with a high proportion of disadvantaged students.