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]
(..) Useful? Maybe. You're on. You have got an hour. An hour? I was supposed to have all night to crack this. And Saito wasn't supposed to be shot in the chest. You have got one hour, now get us something useful, please. - What's that? (..)2010 Inception