Amy Simpson, 18, had been travelling back to a holiday cabin with her friends when the accident happened on Tuesday evening.
And her mother Angela believes the teenager, from Cowdenbeath, Fife, would not have driven back to the site near Loch Awe, Argyll and Bute, if anything was wrong.
After Amy’s car plunged into the River Avich near Dalavich, Argyll and Bute, it landed upside down in 6ft of water.
Her four friends were able to escape but the Edinburgh University maths student died at the scene.
Angela said: “Amy had said to me on the phone about the brakes. They took the car to a garage and they said they were fine.
“She drove the 25 miles from Oban and was nearly at Dalavich. It’s a windy road that goes up and down.
“If there was anything wrong with the brakes at all there is no way Amy would have made it from there to there.
“It has been a freak accident. She was a conscientious driver.
“I don’t believe there was a problem with the brakes but there will be a full examination of the car.”
Amy had recently passed three exams and was due to complete her final exam next month before going on holiday to America for two weeks.
Angela added: “She had her whole life in front of her.
“My gorgeous girl Amy wasn’t just the perfect daughter, she was my best friend, my soulmate and my rock and she made me so proud.
“One day I will make her as proud of me as I was of her.”
Angela praised the efforts of those living near the river, including retired doctor Bill MacRae who was the first to come to the aid of the surviving girls.
She said: “The police who dealt with this have been remarkable, every one of them.
“I would like to thank Bill MacRae, the retired doctor who lives across the road from the accident scene, for his assistance.
“I know there was nothing he could do for Amy, but he helped the other girls.”
A spokeswoman for Police Scotland said: “Our inquiries into the cause of the crash are ongoing and we wouldn’t make any speculations.”