Offering a guarantee in a services business is tricky. When a product company offers a guarantee, the customer must return the product in order to get their money back. This ensures that the customer doesn't get the product for free just by exercising the guarantee.
But a services company that wants to offer a guarantee faces the risk that a customer will ask for their money back after receiving the service, and since there's no way to give the service back, the customer ends up getting it for free. To avoid this, most services companies don't offer a money-back guarantee. Instead, they offer a guarantee that provides value in some other way.
For example, at Blue Fish, we write custom software for Fortune 500 companies. Our guarantee is that once we release the software to our client, we will support it for free for three months, answering any questions and fixing any bugs that may be found during that time. Most other companies would charge by the hour for this (even for bug fixing), so our guarantee helps convince our clients that our software will be high quality, since bug fixing is on our dime.
In your company, my fear would be that students (who are often cash-poor) would abuse your guarantee and ask for their money back even if they were happy with the quality of the review. My recommendation would be to offer a free review of a second paper rather than offering to give them their money back. This way, if they are trying to take advantage of you, it doesn't cost you revenue. And your customer will get another opportunity to experience your great service, which increases the chance that they will become dependent on it and will continue to use you in the future.
If it were my company, I'd also limit the number of times per year that a single customer could exercise the guarantee.