The teenager was giving evidence at the fatal accident inquiry into the death of Keane Wallis-Bennett at Liberton High School in Edinburgh in 2014.
She said she had her back on the wall with her legs on an opposite wall when the structure collapsed backwards onto 12-year-old Keane.
The girl gave her evidence to Edinburgh Sheriff Court over a video link.
She said she felt scared the next day when the police came to question her.
Keane died after a modesty wall in the school gym collapsed on her in April 2014.
The girl told the court: "We were just mucking about, having a laugh, with our feet on the opposite wall for a couple of seconds.
"I don't remember what happened after that. All I can remember is the wall falling back."
She was with another pupil at the time.
The inquiry had heard that pupils sometimes did "wall-walking", by bracing their backs against one wall and their feet on the other and going up the wall.
Advocate Gavin Anderson, representing Keane's parents, asked her if they had intended climbing up the wall.
The girl replied: "No, it was just my legs were up in front of me."
She added that she could not remember if the wall fell while she was in that position or just afterwards.
The teenager told the court that her and her friend had been in the position for "only a couple of seconds" and said they were not pushing the wall. She fell backwards when the wall collapsed.
Asked if she had a name for the game, she replied: "No, it was the first time we had done it."
A second girl told the inquiry her leg had been bruised when it was struck by a piece of masonry from the wall. She had been standing up at the time, she said.
Two more girls are expected to give evidence on Thursday. The inquiry has been adjourned until then.
A schoolgirl has told a court how she was leaning against a wall when it fell and fatally injured a fellow pupil.