For those that look at this thread for reference, if your scroll is not approved before your term then you will not get back pay. I am doing the AF dental 3 year HPSP and did not get my scroll approved till November of 2015. The other AF HPSP students in my class had their scroll and commission in Feb of 2015. They commissioned me at the end of November 2015 and told me that I would not be getting back pay and my pay would not start till the second semester. I tried to get them to give me back pay but was told that it just wasn't in the budget. So my 3 year HPSP ended up being 2.5.
Come to find out the person responsible for submitting me for the scroll didn't submit me and one other guy. Moral of the story, don't expect back pay.