Stop stressing! Time is one of those things that children get in their own.... er time. IMO it's a life skill and shouldn't be in the curriculum apart from problem solving activities in Y5/6. If I were you (hope nobody's listening) I would try with the most able group and see how you get on. If they're struggling, forget it. Try again in 6 months time. If the top group get it, try the next group down etc etc. No point forcing something this difficult if they're not fully understanding it.