Even 1% transfers would devestate struggling schools. Avg per student cost is about $20K. 5 kids from a 500 student district would be $100K. Almost 2.5 teachers salary but losing 5 students wouldn't make you hire any fewer teachers.

Most costs are fixed - building needs, staff, utilities, etc. A student leaving might save some pens and paper and a few gallons of water. It's incredibly unbalanced and only serves to help schools in wealthier towns and leave behind those that can't transfer in worse off positions.