There's no way to guarantee a passing grade, but you can increase study hall time to maximize the odds of a player qualifying. Each kid is different, but giving him study hall time when he's already in trouble may be a case of "too little too late".
Also, study hall minutes are cumulative over the course of a grade period. For example, having 10 minutes there for the entire grade period is about twice as helpful as having 0 minutes until mid-terms and then putting in 10 minutes. So be careful with a strategy of skimping on minutes until you see what mid-term grades are.