find out why they aren't. are they to busy with work, sports or friends? do they have a learning disablity?
if its friends, work or sports make them cut back on them. IF it takes takeing away the car, TV , CELL phone etc do it.
don't hestiate to check the homework and get daily letters from the teachers (if the teacher gives a shit).
those that say "you can't" i feel for your kids..