THUMBS DOWN to ROBERT MEDRANDO for seeking to return to the Dallas Independent School District board. Medrano, who was voted off the board in 1987. is a bad dream from the past, a man whose reputation has been shadowed by allegations of diverting public money to his relatives, voting fraud (in his last school board campaign three years ago), and a 1986 conviction for DWI. Perhaps worse, Medrano would be a strident, uncompromising force on a board that already has more than its share of noisy ideologues. The children and taxpayers of the DISD deserve better.