I think your mother's compromise is a good one.

However, another idea is working with your parents, and asking them that, if you show progress, (not fixed grades, but steadily improving grades, that you may buy it now, and start playing it in a week or so on weekends if you start doing better, and that you may continue playing it as long as you show improvement. Then ask that once your grades reach a certain level, like say a B or B+, that you may resume playing it on week nights once your homework is complete. It sounds like your mother may understand you better, and it would be wise to bring this up with her first, and approach your step-father together with the idea.

Hope this helps man. I've been in your shoes before. It will get better, compromise or not!

So it's not costing them money now, but are you going to pay for your Live then? Also your grades are low for some reason. Maybe take notes ask for help from the teachers and raise them up. You'll get the system eventually. Also you can't trade systems if they don't work properly, hell you can't trade them at all as far a I know. Be patient and respect your parents.......