There's no such thing as a 15 hour course...of course.
It's a MINIMUM of 15 hours.
However, using OBA will give you the chance to fly both the lessons and the test with the same person (the examiner) and as the examiner is the school owner it's in his benefit for you to succeed.
He can find his 40% failure requirement elsewhere - at the "competition".
No idea about Tayflight.