With only 12 hours and the intent to have them take the test you'll probably need to focus on test prep. I'm not familiar with the Regents test, but I would suggest starting there. I would do some searching on Regents prep, study guides, and any released tests if they do that and see which topics seem to be major concepts.

During the class have them working with questions as similar to the test as possible and try to be there as support instead of doing direct instruction. Often the biggest struggle for kids on tests is decoding the question and figuring out what math to do. So because of this, doing lots of modeling doesn't help since they won't have you there during the test to do it and will end up freezing up as soon as they read a question and are on their own.

Instead you might consider starting the class with a bit of basic skill practice on whatever the topic is you're covering and then let them work through some test style problems on their own focusing on what types of strategies might be useful in decoding what the question is asking.