What program is the school using? Is it Primary Mathematics? If so, there are workbooks that help teachers with differentiation within a mixed ability classroom. For example, high ability students could be given the Challenging Word Problems and/or Intensive Practice workbooks to do in addition to or instead of (I don't recommend this unless the child already knows what's being taught) the regular workbook assignments.
In a homeschool setting, PM is *very* easy to accelerate, BTW.
If your school is using something other than PM, the none of what I've written here applies, though if it's something like Math in Focus, there is a workbook containing challenging problems available.