selecting more than one record through a javascript event

function defaultparameter()
{
if (document.myform.select_default_list.value == "M_CH")
document.myform.select_school.selectedIndex = 2
}

I am trying to select more than one record in a multiple select drop down list through the above javascript. It works fine for selecting one record. How would i make it point to more than one ? Any help is appreciated