Anyone who says that the OP shouldn't have/shouldn't let the 17 year old ride the ninja 250 forgets that if a 17 year old wants to ride a sport bike, but wants a safe bike to start on, the ninja 250 is one of 2 realistic options on the market. (I don't even consider a Hyosung here)
As someone who started riding a ninja 250 as a 17 year old kid, I'd say this is THE BEST option to get a high-school kid riding, besides a dirt bike. That being said, make sure that said high-schooler has taken the MSF course before even thinking about taking a motorcycle out on a public roadway. That was my course of action; Gear, MSF, license, insurance, THEN motorcycle
Don't restrain someone from riding when all of us here know how great riding is and how great this bike is for beginners. Everyone has to start somewhere