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This isn't really a question any of us can give you advice on. Temet nosce: you've had four semesters so far. You should be able to judge if you can add a substantial time and effort commitment onto the credit hours you've been taking. We can't really tell you "yes you can do it and still take 15 hours" or anything.

Take 12 in the spring to have more time closer to the test.

I am in a very similar situation to you. I'm planning on taking June '14. I am doing 15 this fall and 12 in the Spring (and quitting my part time job in the spring too) worst case scenario you do too many hours in the Spring and you have to wait until Oct '14, which still keeps you on schedule