The court also heard how Micaela had a history of self-harm.
One night about a week before the tragedy, she was so upset that she could not get to sleep, Mr Gardiner said. The pair stayed up all night creating a Facebook page called Rumour HateIt, on which they bogusly posted that Micaela had had an abortion after being six weeks' pregnant.
Both made a pact that if either of them felt suicidal, they would phone the other "no matter what".
Micaela had been looking forward to her 18th birthday party in September, and had written most of the invitations.
On July 23, she went to Mr Gardiner's house, where they had tea and watched a movie before going to a party in Tokoroa.
Mr Gardiner said Micaela was in a "good mood" and drinking ready-mixed drinks, although at one stage he suggested they should go back to his place and drink there. "I just had a vibe things were going to go wrong."
The two later argued after she asked if she could bring a boy to his house, where she was supposed to be staying that night.
Another friend, who had seen Micaela hugging friends and saying, "I love you so much, you're my bestie", later received a text from Micaela reading, "So much for besties", when Micaela discovered she had left the party.
When she texted back inviting her to stay at her cousin's house, Micaela declined and asked her to tell her family that she loved them.
Between 3.37am and 4am, more friends and family received text messages, including "I love you, don't forget me, tell my friends I love them".
Later that day, Mr Gardiner discovered a missed call from Micaela at 4.30am. At 12.30pm, police found her at David Henry School. She was pronounced dead at hospital soon after.
Speaking at the inquest yesterday, Mr Gardiner said he should have told somebody she had been making suicidal comments. Soon after the death, he took the Facebook page down, saying he "didn't want anyone to get harmed because of it".
Until yesterday, her family believed Micaela had been bullied on Facebook - her mother recounting how she had been in tears over the post about her.
Mr Gardiner said: "All of our friends know that it was me and her that made the page ... I didn't know that the family hadn't known."
Coroner Wallace Bain, who reserved his findings, said the case highlighted concerns around texting and social media.
Micaela's father, Les Pinkerton, hoped the tragedy would send out a message. "For me, it's speak out, kids. If your friends tell you anything about suicide, speak out ... It's no good waiting until they're dead," he said.
Mr Pinkerton believed suicide was a "tapu word" which caused people to back off. "We've got to change it. It's killing our kids."
SUICIDE: WHERE TO GET HELP
If it's an emergency and you feel like you or someone else is at risk, call 111. Or call Youthline 0800 376 633, Lifeline 0800 543 354, Depression Helpline 0800 111 757, What's Up 0800 942 8787 (noon-midnight).