This level's Secret Exit is one of the hardest to reach.
First, in the white, use a P-Switch and quickly head to the 5th & last door to the right.
In the next area, pick up the P-Switch you find there, and take it to the other side of the same area. The Secret Exit is inside a small "cage" of sorts above you, and there's no way you can fit under its opening by flying.
Use the ? Block nearby to start a coin path and try your best to make it line up with the small opening, or at the least enough that you can crouch jump into it.
Then use the P-Switch to turn your created path into brown blocks that you can step on. Somehow, I perfect got this path right where I wanted it to be without having to crouch jump into the hole from below. AWESOME!
![](https://i.ytimg.com/vi/Ja8qSFpPKx0/mqdefault.jpg)