PLAINVIEW - Residents of an upscale gated community in Plainview were in for a nasty surprise when they opened their school tax bills to discover a double-digit increase.David Pollack has been living at the Hamlet for the past nine years, but over the past year, he says his school taxes went from up from $15,261 to $17,153, a difference of nearly $1,900.Thousands of Nassau homeowners are seeing unexpected hikes in school taxes which stem from the county's complex assessment system. Experts say when property values fall, tax rates must be increased to make up for the loss, and as a result, some homeowners are paying more than others.School tax bills make up 60 percent of homeowners' taxes. Last year, County Executive Ed Mangano (R-Nassau) announced a new transition team that would correct the assessment mistakes.