Don't be discouraged Hedgetrimmer, the gang here is sensitive to their public image and their legal liabilities. I'm more concerned with your using a grinder than what you make on it. Be sure you get proper instruction on power tools before you use them. I'm glad you appreciate the importance of eye protection.
A special warning about rotating tools--they love to grab onto cloth. Drills, sanders, grinders, milling machines, lathes... Rotating machines are fiends for grabbing cleaning rags, shirts, long hair, even gloves. I've had this happen several times when I try and hold or clean something I'm working on. I came close to losing my finger tips once when a grinder grabbed the tip of a glove.
Making things is very educational. It is best to find things that you can make that you can show off to your parents and teachers. Suddenly you're clever and creative in their eyes. I made knives, spears, crossbows etc when I was young. I just did it openly and had a place to experiment with them. They were always "tools", "experiments", and "explorations of ancient technology"--NOT weapons. Designing and building things is one of the ways I got into my career as an engineer. My best friend was even more that way than I was, he's now one of those guys who controls deep space probes for NASA.
If you want an interesting challenge (with parental approval of course) try making an air gun. Brass tubing, copper pipe, gas valves, bicycle pumps, epoxy... throw a bunch of these components together with a lot of ingenuity and you could make your own paintball gun. It wouldn't be the worlds best, but it is a lot of fun and a tremendous education.
If you can get reasonable approval for experimenting with knife making, experiment with heat treating. Take your hacksaw blades and bake them in the oven at around 200 degrees Celsius for an hour or two and see how much tougher they get. Play around with that sort of stuff and see if you can make a leaf spring out of a hacksaw blade.
Go over to our Shop Talk Forum and learn some of the mysteries of steel. Don't ever let people discourage you from experimenting, just be safe and stay on the good side of your parents. Find ways to work that issue. Your parents can make great allies if you convince them that you are going in constructive directions and learning skills and technology. If you are real good or lucky they might even buy you tools and supplies for your projects. See if you can fit some metalurgy into a school science project.