You don't say what your kids' ages are, but I think removing tv & computers would be a logical consequence for not doing a good job managing school time. If the work they are doing doesn't improve, then they don't get the privilege of screen time back.
However, sometimes using things as an incentive ends up making those things even more valuable to the kids, iykwim. So they might end up even more addicted if you use screen time as a reward.
Lots of people have luck with a house rule of 'No tv til the public school kids get out of school' - maybe that would be worth a try?
The other thing I might try would be to just come iin one day and say, "Woudja look at that - cable's out! No tv & no internet for a while. Oh well!" This way they can have a screen-free stretch of time, but you're not getting into a punishment/reward dynamic with them. You decide when/if the screen options should come back into play. They will have to detox (kinda like deschooling, if you took them out of ps originally), and things will get worse at first, but you might be shocked by how much they change after a week or two!
Ok, and the last thing - if nothing else, try to at least get the tvs out of their bedrooms. Right there I think you'll see a big difference in their attitudes. Again, though, I don't know how old they are, so maybe this advice doesn't apply!!