How about pay for the motorcycle safety course yourself, and show your parents that you want to have fun, but you want to be safe. Insurance should be cheap, I pay $125/yr for my '02.

I'm sure if you let your parents know you want to be a safe driver they will be less hesitant to let you have one. And if you take the course and get your endorsement, and your gear, then you're even closer. The safer the gear, the more they will feel better about it. And I'm sure the fact that it's a 250 is better than being a 600 or larger.