That's because your parents are probably using the RIGHT approach and trying to teach you the value of money and earning things for yourself, not simply getting something just because you want it and you asked nicely for it.
If you really want them to buy a console for you, you need to show them you've earned it. You need to negotiate why you need it, why you believe you've earned it, and how you'll be able to use it appropriately and not simply become a 14-year old punk kid that sits infront of a TV playing video games instead of keeping up your grades and all that kind of stuff. Offer to do things around the house and all that stuff... dishes, garbage, all the stuff they'd normally do, and show them you're willing to work to get what you want. You might not get a "Yes" right away, but you might get a "Well... if you show you're willing to do it, then we'll get it... We'll see.", which is better than a straight out, "No".
Another thing I'd suggest, is your own self-discipline when it comes to your school work and everything... Something like, "If my grades start to slip, you can take it away from me", or "If my grades start to slip, I'll unhook it myself and I won't touch it until I get my grades back up to where they are". That being said, if you're passing your classes by the skin of your teeth, you need to open with "If I get my grades up to (insert grade here), and keep them there...", that will at least show them that you have a goal in mind, and the console would be the reward.
The other option... get a part-time job and earn it yourself. That's all I can say about that.