American Opportunity Tax Credit Act of 2013 - Amends the Internal Revenue Code to replace the Hope Scholarship and Lifetime Learning tax credits with a new American Opportunity Tax Credit that: (1) allows an income tax credit of up to $2,500 of the qualified tuition and related expenses of a student who is carrying at least one half of a normal course load, (2) increases the income threshold for reductions in the credit amount based upon modified adjusted gross income, (3) allows a lifetime dollar limitation on such credit of $15,000 for all taxable years, and (4) makes 40% of the credit refundable. Allows an exclusion from gross income of any amount received as a Federal Pell Grant.