Lynne Peterson said her daughter graduated yesterday with the rest of her year 13 classmates, but no parents attended the ceremony because there was no room in the school hall for them.
''I've been waiting 16 years for [the graduation] to happen.
''I'm disgusted. I would have liked to have been there to see it.
''I feel like I've missed an important part of my daughter's education.''
Mrs Peterson believed she was one of many parents who were unhappy about the situation.
Principal Judith Forbes said the annual event was the school's last full assembly for the year, and with every pupil and staff member in the school hall, there was no room for parents.
''Parents are invited to attend the school's prizegiving instead.
''It's certainly not a snub to parents. We just can't fit them in for this particular assembly.
Mrs Forbes believed it was important for the whole school to see the graduation - particularly for the junior pupils to see the seniors graduate.
''It's a good role-modelling session.
''It's the school family gathering together for the last time.''
The event was also a time to announce the head pupils for the following year, and a time to recognise teachers who had won awards.
This year, five teachers won National Excellence in Teaching Awards, she said.
While Mrs Peterson said she would attend the school's prizegiving ceremony next week, she felt it was not ''the main event'' in her daughter's education.
''I know I can go to the school prizegiving next week, but the graduation has more finality - it's saying you're leaving school.
''To me, that was more important than the prizegiving because she may not get a prize.''
Mrs Forbes said the school had not received any complaints from parents about not being invited to the graduation ceremony.
If there were official complaints, she said the situation could be reviewed.
''This is a during-school-time event - the only one parents are unable to come to.
''But it is possible we could review the situation.
''We are always reviewing our processes,'' she said.
