Here’s what I have done in Middle School, but you could also adapt it to HS:
Start the class with bellwork for the first 5 minutes. Make it old review work. Allows you to take attendance and you can circulate around and help those that need it.
Then transition to a number/pattern talk for EVERYONE. Everyone has equal access to the information in those (usually?) –> like visualpatterns.org, or an esti-mystery from Steve Wyborney (or any of his other cool stuff).
As for the lesson itself, it sounds like a “lab” or “intensive” class that the kids take in addition to their regular math? Or are you still beholden to grade-level standards?
What I am working with my lab teachers with is identifying the core standards/skills that transcend grade-level. And those kids ALWAYS struggle with them. Stuff like:
Propotional Reasoning, Identifying/Combining Like Terms, Writing Expressions/Equations through Articulating Patterns, and basic number facts (not just drilling, but how can they figure out with proportional reasoning or open number lines how to get an answer when they’re stuck).
And then do a quick task about it or whole-group-instruction, then break them up to build and model those tasks.
I would also devote at least once a week for them to group by grade level and you can devote it to homework time.
Any of this helpful?