There's no easy way to debug your scripts on ScraperWiki, unfortunately it just sends your code in its entirety and gets the results back, there's no way to execute the code interactively.
I added a couple more prints to a copy of your code, and it looks like the if check before the bit that assigns data
if len(tr.cssselect("td.ranking")) > 0 and len(tr.cssselect("td.rankingname")) > 0:
doesn't trigger for "York University" so it will be keeping the int value (you set it later on) from the previous time around the loop.