"When I read it I thought I hope it's not the same Theo Williams that we know and it's only been during the day that we've confirmed that it was."
Even though he finished school at the end of 2017, Hassall remembered him as a "really, genuinely lovely young man".
"I recall him being very hard-working and he succeeded academically because he worked hard.
"He was part of our Year 13 mentoring group so he had a leadership role in the school and supported the younger students in that role."
She recalled him being into languages and academic subjects at school - saying his later love for adventure sports didn't "show itself through the school world".
Hassall, had been offsite on Monday, but planned to speak with former teachers and reach out to his family on Tuesday to offer the school's support.
"Our staff will be attending the funeral. I will definitely be in touch with the family offering our support and offering anything the boys can do to ensure the service is a fitting memorial for him."
A former classmate remembered Williams as "being a great mate".
"... you lived your life by having the best adventure you could and I hope that your adventure never ends. Thank you for all the good times at HBHS," one wrote on social media.
New Zealand Skydiving School has also expressed its sadness at the death of the former skydiving student
The school's operations manager Fiona McLaren said the 21-year-old had a passion for the extreme sport that claimed his life.
Staff and students had been left heartbroken by the tragedy.
"Theo was a dedicated student who loved skydiving, yet he was a gentle but confident young man," said McLaren.
"He was often described as an 'old soul', was extremely well liked by all students and instructors, developing very strong bonds with his fellow classmates."
Kiko Afuie worked with Williams at Air New Zealand in Wellington until he left in early 2020 to complete his skydiving training.
Afuie said he was an amazing guy, who loved the outdoors and was always on the go.
Another friend has paid tribute to Williams on social media as the "funniest, most heartfelt, most dedicated and most understanding" person.
"I am so privileged to have met you. You made the world a better place. I will miss you always and love you like a brother."
Emergency services were called to the Bay of Plenty airport on Kittyhawk Way at 10.33am on Saturday after reports that a parachutist had crashed.