Here's your plan. Take a dollar or two to school everyday (If it's too suspicious, try every other day) and then give it to a trustworthy friend to hold on to. After about 25 days, you'll have enough "hidden" money to buy a visa card, then you can use that to buy TF2. Secretly.
Also, how could you try to buy TF2 at gamestop without your dad knowing, but you can't leave your house with $25?