Ironmom, Aleks.com has an assessment that comes with the program starting with third grade. They usually have a one month free trial,, you could start there.

Singapore math offers a free assessment you can print out and give. It's geared towards their learning program which I find is advanced compared to US standard education. Ex., multiplication starts at level 1, 1b.

Here is a link, scroll down for the links to the assessments:

http://www.singaporemath.com/Placement_Test_s/86.htm