One of Sydney's newest schools, The Ponds High, is full just three years after it was built and before it even has years 11 and 12, prompting concerns that the NSW Department of Education has underestimated swelling enrolments in public high schools.

The north-west school was designed for 1200 students when it opened to its first cohort of 180 year 7 students in 2015. But enrolments have been soaring and when school returns at the end of the month, The Ponds High will get 320 year 7 students.

P&C president Roland de Pree (right) and Blacktown City Councillor Chris Quilkey with parents and students at The Ponds High School. Credit:Wolter Peeters

Enrolment figures show that the school grew from 180 students in 2015, to 453 in 2016 and 815 last year. The oldest students will go in to year 10 this year, with two more year 7 cohorts to start before it has its full complement of students.

The school's P&C president, Roland de Pree, said the school needed an urgent infrastructure plan to deal with a looming overcrowding crisis.