I think for middle school age, if they show an ability to handle some things, then you want to help them increase it. Not all kids have the same development capacity, but if they're tracking average development, you can probably think of it like being an adminstrator rather than a teacher (this is what I think about with the older kids). Letting some of them go with no guidance can be too much (some will thrive). But they should be able to work day to day within a system. I would try sitting down and working out a complete schedule (windows calendar or the gmail calendar, etc, can even do this for you on the computer). Have them look at how much time there is for schoolwork. Then look at the average amount of schoolwork they get a week. Help them figure out what is optimal for them - do they prefer an hour each day, or a half hour weekdays but a long block (say 2 hours) on weekends? Use the syllabus provided by the teacher to put all big assignments on the schedule - it's a lot more stressful to vaguely know there's going to be a project at some point and maybe there will be a meet or a concert or whatever. It's better for them to have a concrete idea of what projects are due when and identify any conflicts ahead of time.
I wouldn't necessarily schedule the exact time they will do every bit of homework - I feel that's too much and hard to hold to, then they'll feel guilty that it doesn't work. But I would encourage them to look at the big assignments and break them down into a timeline - I will do the thesis or experiment for this project by the end of this week, and finish the presentation by the end of the next. Otherwise, I would just help them see what time they have, and what they need to do with that time.