What I did about the math...
The math difficulty that I was describing below was more a problem with the children's division skills than it with their ability to calculate an average. You see, I had all the kids walk as fast as they could for the length of the hallway (distance = 42 m) while a friend timed them. They came out with a number in seconds and hundredths of seconds, which we rounded to a whole number to keep things relatively simple. Then they divided distance by time. This is where the problem started - many of the kids are still accustomed to dividing and getting a remainder (2 r 4 m/s) rather than continuing to divide and getting a decimal answer (2.2 m/s). But if you have a string of numbers like 2 r 3, 2 r 7, 3 r 1, finding the average is really tricky, and the kids rightly asked me how to do it. Most knew how to compute an average using whole numbers, they just didn't know what to do using numbers with remainders.
So, I decided that the simplest solution, and best for their long-term ability to do useful things with data, would be to learn how to divide out to decimal places rather than using a remainder. I taught the whole class how to graph their data on a distance-time graph, then I asked those students who had used remainders or wanted a brush-up to sit at the front three tables, and moved the kids who already knew how to divide using decimals to the back to start making their graphs. I taught the front group the steps to divide out to tenths of a m/s, we did a couple together, and then they spent the rest of the period recalculating the speeds.
I'll see when I get their lab reports whether or not it worked, but I think it did for most of them. I realized when I thought about it more that they already knew the steps for long division, and they really just needed to be shown what to do with the decimal point and how to add zeroes so you can "bring down the zero." We talked briefly about how 42 is the same as 42.00, and then we just went ahead and started dividing.
I see it as an important part of my job to incorporate math into science class, but I really prefer to let them learn new ideas in math class, and just to reinforce what they've already learned and to demonstrate the ways that scientists use math in my class. I don't feel like that's a cop-out, I think it's just realistic.