Grades !!!!!

I am so struggling with parenting my teenage sons. If they were 6 years old I would spank their behinds, ground them from cartoons, or make them sit in time out. I can't do ANY of that with my teens. Two of my teenage boys are failing. BAD. Not by a few points but my 20 and 30 points. I have grounded them. Their attitude: Mom I'm gonna bring it up. My teacher hasn't put in my grades. Its not my fault. Everyone is failing that class. I am so SICK of it. I guess they think I want them to make good grades for me. I can not get it through to them that if they fail high school classes they will never make it at college. Let alone get scholarships to pay for college. There is not enough support from the school system. Why would they let these boys play sports? I mean even if they break track records and win at state in wrestling what is that doing for their life? They can not make a living at wrestling or track or basketball. They are not THAT good. So what should i do? One boy does seem to get it. He is just so la de da. Oh I'll get it up. The other could care less. When told he was grounded and couldn't go anywhere he went ballistic. He knocked things over, cussed and said he was going to fail everything. So how can I get it through to him. If I let him suffer natural consequences like failing a class, what is that teaching the other kids. Just let it go. I Can't. I just can't parent like that. I am trying to parent and teach responsibility. What can we expect out kids to do in this world if they don't show some kind of understanding that the world doesn't bow down to them. No one in this real life will make exceptions for them. If you have any ideas I'm open to them. Therapy is already involved. Mentors are already involved. I tried to get teachers involved, they made promises that they did not follow through with. They did a little so they could stay in the sport and then just let it go. What can I do to make them understand? Thanks for letting me vent.