"She was doing well and she seemed to be very positive around the school in terms of her attitude in class, so it was huge surprise to hear of her sudden death," Mr Hullena said.
The teenager, affectionately known as Gabby, was in Year 12 and studying towards her NCEA Level 2.
On a Facebook tribute page, a message read: "Nobody knows why she did this, yeah she was bullied, she has been through so much throughout her life."
Mr Hullena said the school had not been informed of any bullying.
"I think there might have been a couple of relationship breakdowns recently, in terms of friends, and words might have been said."
Mr Hullena said the college, which has a roll of about 300, was very proactive in picking up bullying and there were clear processes in place to deal with it.
The school was offering counselling for students affected by Gabrielle's death and was operating alongside the Ministry of Education and social services from the Wairarapa region, he said.
WHERE TO GET HELP
If you think you or someone else is at risk, call 111, or contact:
Youthline 0800-376-633
Lifeline 0800-543-354
Depression Helpline 0800-111-757
What's Up 0800-942-8787 (noon-midnight)