A dilemma that i think is faced by all teachers of all subjects the world over! I attended a lot of meetings back home that were essentially looking to find the best solutions for this question.
One thing that i think everyone can agree on is that you cant hold the entire class back because some students are having difficulty.
I think what is generally encouraged is'differentiated' work, which means adopting strategies to help students that are having difficulty accessing the curriculum. This could be for so many reasons, mainly learning and behavioural difficulties. One suggestion is to do the same activity but prepare different tasks to cater to differing levels of ability. Granted, this means increasing your workload but aside from completely ignoring the lower level students, thats what needs to be done what ever action you take.
You can google differentiation in teaching to get some ideas.