I second Singapore Math, both the workbooks and the Challenging Word Problems. I also love Zaccaro's Primary Grade Challenging Math. That's what we will use to homeschool DS5 next year (1st grade on the paper, 4th grade math).

There are placement tests for Singapore Math, you may want to look at them first. Kriston is right about the curriculum being ahead by half a year.

Good luck dealing with the school.

LMom