Wisconsin Taxpayers Alliance, via a kind Senn Brown email:

In 2010-11, a record number of students took advantage of Wisconsin’s open enrollment program to attend school elsewhere than in their own district. The 34,498 participants was 8.1% higher than in 2010 and nearly five times higher than in 2001. Open enrollment numbers varied widely, with 13 districts experiencing net outflows of more than 10% of their student populations and 34 with net inflows of similar magnitude. These findings are detailed in SchoolFacts11, the annual reference book from the Wisconsin Tax- payers Alliance (WISTAX) that provides, for every school district in the state, a wide range of information on enrollment, finance, staffing, and test scores.
In 2010-11, 4.0% of Wisconsin’s public school students attended a district other than their own. Dover (26.2%) and South Shore (23.0%) both had net outflows (students leaving less those coming) of more than 20%. Eleven other districts (Florence, Mercer, Neosho, Palmyra-Eagle, Richfield, Stockbridge, Twin Lakes, Washington-Caldwell, Wheatland, Winter, and Wonewoc-Union Center) had net outflows of over 10%.

Related: Madison School District 2009 outbound open enrollment survey. Much more, here.
Student counts drive a District’s tax and spending authority.