function onEdit(e) {

  var editRange = { // B3:AL13
    top : 3,
    bottom : 12,
    left : 2,
    right : 38
  };

  var range = e.range
  var sheet = range.getSheet();  //Get sheet from which the event occured
  var name = sheet.getName();

  if (name != "Section 1 Homework") return; //Exit if sheet is not a sheet where cells should be changed

  // Exit if we're out of range
  var thisRow = e.range.getRow();
  if (thisRow < editRange.top || thisRow > editRange.bottom) return;

  var thisCol = e.range.getColumn();
  if (thisCol < editRange.left || thisCol > editRange.right) return;

  //Exit if the cell is not blank
  if (!range.isBlank()) return;

  //Set the cell value to the default formula
  range.setValue( "=if(INDEX('Section 1 Attendance'!A1:AL12, " + thisRow + ", " + thisCol + ") = \"A\", \"A\", \"\")" )
}