They probably need to change their diet. If they eat heavy foods full or carbs and sugar them they are going to be sluggish and tired. They should also go to bed early and get up early. If they can't then they aren't motivated and you should probably let them go. Also, if they drink a lot (or do drugs) they should cut down on that in a major way.
I'd say the biggest problem though is they probably aren't motivated. They likely don't see much benefit, so they don't care and they show up when they feel like it.