To get to an understandably traditional K-12 head-count (not full-time-equivalent) number even I could grasp, I subtracted all of the other categories except the ungraded 249, and the remainder is 104,559 - 2,612 = 101,947, which is 9.952 heads under the census number for the school-age cohort. The not-in-public-school numbers include those in alternative-ed venues, ranging from private to home-schooling, numbers which are partially underground by parent choice. (Think Karen Maple in a Vermont jail for defending that choice). It works out to roughly 9 percent of the total. At the end of the decade, the numbers have shrunk.
The 2009-2010 numbers are 5-17, 93,801; adjusted-as-above K-12 enrollment, 91,239 - 4,920 = 86,319. The missing 7,482 are presumably being alternatively- or home-schooled; that's roughly 8 percent. The non-public sector was larger in the past, when the parochial system was more widely used (partially because tuition fees were remarkably low) but today's 8-9 percent of the non-public enrollment represents a stable (in-Vermont) and recently increasing (national average) market share being diverted, by parental choice, away from the once-traditional K-12 format, perhaps (anecdotal evidence only) because of dissatisfaction with shrinkage in achievement and enlargement in ideological content there.
Since this is an opinion column, here's my opinion: the population shrinkage in the age-cohorts representing the peak work-force, wealth-creating years is more serious than the population shrinkage in public education. To what extent middle-age flight equates to middle-class flight, the stats don't tell us, but the state's low (by national standards) unemployment rate certainly suggests that unemployed job-holders and -creators don't stick around where the in-State economics, present and future, have been tilted against them.
As reported (Dec. 28, 2010) from U.S. Bureau of Labor Statics data: "At the end of 2009, Vermont was showing net job loss for the decade." Maybe the inmigrating 20-somethings are mostly trust-funders?