you're a salaried teacher and you don't pay any taxes? are you sure that you're not confusing tax liability with the amount that you owe(or get back) every April? you do have taxes taken out of your paycheck, right?
in other words, assuming you have fed tax deducted just like everyone else, come April tax time, if you've paid all your taxes and you don't owe anything, or if you've paid more and you're getting some back, that *DOES NOT MEAN* you have $0 tax liability. It just means you've already paid your tax liability. For example, if you make $50k, and let's say the fed takes $1k per month out of your paycheck. At tax season time when you file your taxes, if your tax liability is $12k, well you've already paid $12k so you're dead even. You don't owe anything, and you don't get anything back. BUT YOUR TAX LIABILITY IS $12k, not $0. thus you can claim the credit and get $7500 back. If your tax liability for the year ends up only being $5k for whatever reasons (maybe you have lots of deductions or have lots of cash and donated tons to charity, lol), yet you paid $1k a month for 12 months (you paid the man $12k total already though paycheck deductions), then you get $7k back for overpaying, and you can still claim the credit and get that other $5k back too. it's capped at your max liability (aka you can't "make money" from this as it's a tax credit, not a refund).
(and if you really overpay $7k every year you should add some withholding points to your W2 so you don't give the government a free loan through the year!! lol)
besides that, the only way you can claim this credit that I can think of is to create tax liability. make money. how do you do that without getting a new job? well for one you could cash out stocks (profit is taxable), retirement account withdrawal (not a loan; you don't pay taxes on loans; also if you contributed pre-tax dollars then the full amount you pull out is taxable), etc. up to the amount that you would end up owing $7500 at tax time. let's say your tax bracket is 20%
(i'm just throwing numbers out here, if your tax bracket is really 20% then you do have liability but I'm just trying to prove a point here), then if your liability is really $0 after deductions, in order to owe $7500 you have to make an additional taxable ~$37500. 20% of $37500 = $7500. *****
note: i'm not recommending withdrawing from your retirement account. i'm just saying its possible. in fact, pulling out $37500 early is going to cost you way more than $7500 in the long run due to lost investment growth. If you really do have $0 tax liability
(perhaps you could enlighten the rest of us here how to do this LOL), you *really* need to speak to a financial advisor about the right way to create enough liability to get back the full $7500 if that's even possible.
FWIW I know there are retired people that make $0 and have $0 liability. But they might have $5mil in stocks. so if they wanted to get the $7500 back they'll just cash out a portion of their stocks (probably around $37500 in profit's worth), in order to owe $7500 at tax time. then they can claim the credit and get that back.