If your tank is only half full of water, I'd say you should focus on filling it all the way and doing an inexpensive above tank basking area with some plastic and stuff. That way you can wait for a while before upgrading the tank, and you will be a little older and maybe save up more of your own money.
Since you are so young, you would need your parent's help with this, but I always say look at Craigslist for stuff: a canister filter, which would also free up room in the tank.
This way, it is more like remodeling the house you have, instead of convincing your parents to buy a new house!
I think a full 55 gallon with an above tank basking area wouldn't be too bad for a 6 inch turtle, especially if he isn't growing that fast and you can upgrade to a bigger tank in a year or two?
What do you think about that?
Also, the new tank can eventually be bought on Craigslist or with the dollar-a-gallon sale (does not include lids) at Petco (a few times a year). Again, don't try to do Craigslist transactions on your own, I don't want to send an 11 year old to a stranger's house!
Or do what litefoot suggested and switch to a stock tank, if your house allows for it and your don't have dogs or cats!
Also, you shouldn't need to be picking up and dumping the tank to clean it! Get a siphon and do partial water changes, putting the dirty water into a large tupperware. That should be easier and safer...
Lastly, it's awesome that you are so into herps at such a young age. Even though your parents won't let you upgrade your tank right now, it's still neat that they let you have all these critters. Maybe soon you can look into volunteering or interning at a zoo or animal hospital or something- get you more time and experience with herps and other animals, without having to grow your collection right now! Good luck