Parents are concerned that children who have trouble in school will 'fall through the cracks' in the school system. [=will not be given the help they need; will graduate without anyone noticing that they are unable to do the work]
The program is meant to help workers who may have 'slipped through the cracks' [=who may have not been included in other programs] because of their age.
You ought to 'get cracking' [='get going; get moving'] on that assignment. [=you should start working on that assignment right away]
(..) - You good? - Right here. All right. Don't rush. We've got one chance to fuel their plane, so let's do it right. If you ain't a crack shot, don't aim for the head, just get Zeke on the ground. Remember, "Spine's divine, those knees they work just fine!!" Let's go, lights out!! (..)2013 World War Z