Challenge of Pacing a Bimodal Group

I've been thinking about ways to address the bimodal performance that my Grade 8 kids exhibited on the last exam.

Good news: I am getting a lot of motivated kids to come see me during lunch for extra reinforcement to fill in the gaps of their learning.

Bad news: I think the bimodal performance is a result of my not very effective attempt to keep slower and faster learners paced similarly in the same class.

Imagine a typical class: I give kids handouts to work through at their own paces. The problems are scaffolded, so they get harder as you go. Kids are helping each other through the assignment, although there is a healthy amount of struggling happening per kid, and that means that each kid is moving at a different pace. The class works steadily for the entire period, and at the end of class, some of the faster kids finish the worksheet and grab a new one from me, while the slower kids are about two thirds of the way through with the worksheet. I consider assigning the last third as homework, but I also consider the fact that it's the hardest part of the worksheet, so I change my mind and tell the class that they can continue working on it the next day.

Fast-forward to the next class, or two classes from now. The faster kids are now a couple of worksheets ahead of the slowest kids. Instead of giving them pure algebra practice, we now are attacking the problems from all angles -- word problems with non-trivial context. My hopes are that this way, all kids are benefiting from the new assignments (whether it is additional algebra practice OR additional exposure to new contexts). I give the most advanced kids an extra challenging assignment, and I decide that it's definitely time for all of us to sync back up on our pacing, so I assign close to an entire worksheet as homework for the slower kids. The slower kids feel punished for working more slowly than their peers, even though I explain that we need to get everyone more or less on the same page now.

In the end, my slower-working kids are the ones who should probably be doing extra problems just to keep up with the material, but the reality is that they need to spend extra time at home just to finish the regular assignments that everyone else can finish in class. Naturally, their learning results are not where I want them to be, because my pacing is at least somewhat dictated by the fact that half of the class is absolutely mastering quadratic operations forwards and backwards.

How do I differentiate my instruction enough to fix this??