He described how the touching occurred after she'd slipped into the bed he was sharing with another boy, lying between them.
He supported evidence from other boys who had testified that earlier in the evening Purdie produced a sex toy which they'd played with and that she'd watched pornographic videos with them.
He said the day after her sexual advances to him Purdie asked him to become her Facebook friend and when he accepted she began messaging and trying to call him.
He claimed one message said she'd "drop" him which he took to mean give him a hiding.
"I didn't feel safe and didn't want to talk to her," he said.
Messages continued to arrive saying the coach wanted to talk about what happened the previous night.
"I didn't want to talk about it, just wanted to forget about it, I was freaked out, it was so weird."
He told prosecutor Ngaroma Tahana he was 100 per cent sure his coach had touched him and, after removing her bra, put his hands on her breasts.
When the trial opened on Monday Purdie's lawyer Harry Edward disputed the claim Purdie exposed the boys to indecent material, saying it was circulated by two of their number.
The trial continues.