Yes, if the co-op was (as it appears from your question) a school-approved program for which academic credit was granted, then you were a "more than half-time" student for the entire year.
If you have already filed, you should file an amended tax return claiming all of the qualified tuition and fees paid for the year. I am certain your school will have charged you fees, and probably tuition) for those 12 hours granted for the co-op.
TurboTax will guide you through correctly entering your new data
and printing Form 1040X. You may also have to amend your state income tax
return if you have a state income tax. Please note that amended tax
returns must be filed by mail.
For more information, please see TurboTax
Guide - Amending Your Income Tax Return.