Yes you can amend your tax return to claim the American Opportunity Tax Credit.  See How to amend (change or correct) a return.

don't apply the first year you get out of high school. You only have half a year of college  tuition to work with .

Yes, if you are eligible for an education credit or deduction, but did not take one, you can amend your return and claim one. 

The American Opportunity Tax Credit can only be used 4 times, so it would be smart to think about the years of schooling you have left and the anticipated amount of expenses you can claim each year. 

The other credit is the Lifetime Learners and there is a Tuition and Fees Deduction as well.

IRS Pub 970 explains the differences between the credits and the deduction, and also shows examples of how the credits, scholarships, and 529 distributions can be allocated to get the best result. 

The Education section is an area where advance planning can pay off. 

IRS Pub 970

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