I feel ya'. I struggled with my grades starting back in 4th grade - I would start the year off strong, then plummet. For nearly eight years I was constantly lectured by my teachers and my parents about my grades - and they said pretty much the same thing, in a different way. I had to go to summer school, and take a few classes again after school a couple times, and I had to pay for the fees. All I can say is that, as long as you can keep your grades at passing levels, then you'll eventually graduate - and once you graduate, nobody can say anything, because you'll have done it.
My parents can do the whole "on-off" thing pretty darn good too - I guess it's a parent thing.