There's almost no way to safely drag a knee on the street, particularly on a Ninja 250.
- If you're leaned over enough to get your knee down, you've generally left no traction reserve available with which to change your line or brake when something unexpected pops up. And it will. (Which brings up the second form of traction: the kind used on broken bones.)
- Racers will put their knees on the ground to measure their clearance. That is for sure. If you are riding on the street such that you need to know just how much clearance you have, you are pushing too hard and need to pull back a couple of notches.
- Unless you are hanging completely off of the bike, a street stock Ninja 250 will generally drag hard parts, such as the centerstand, before you get far enough over to reach the ground with your knee. If you drag those parts hard enough they will neatly lever the rear tire off of the ground, causing you to sing "I believe I can fly".
- What you need to focus on is having a relaxed upper body, rolling on the throttle through a turn, intentionally countersteering, and leaning with the turn. When you get to the point that you are starting to shift your body into a turn, the knee will hang out some on its own with no conscious effort on your part. It is the direct result of your body being moved over in the seat.
- The guys who race 250s do get knees down on the track, but their bikes have been modified with stiffer suspensions, better tires, removed center stands, and so on.
Best answer: Ya wanna learn racing techniques, get a bike that's set up for the same and get yourself to a good track school. Once you've been fast on a track you'll never want to ride so hard on the street; there simply isn't any comparison in terms of fun or safety.