Getting class in salesorder according to custom form

Hi All,
In salesorder form i got class with so many values in it.
When i am doing a certain form i should get only one value among those list in class.
I wrote a client script on Fieldchange but it seems not working..
So please suggest some solutions.



function getonlyonenameinclasslist(type, name){


if (name == 'entity') {
  //alert('in condition');
  var formid = nlapiGetFieldValue('customform');
  if (formid == "107") {
   nlapiSetFieldValue('class', 1);
  
  }
  if (formid == "108") {
   nlapiSetFieldValue('class', 2);
  }
  if (formid == "109") {
   nlapiSetFieldValue('class', 3);
  }
  if (formid == "110") {
   nlapiSetFieldValue('class', 3);
  }
  if (formid == "111") {
   nlapiSetFieldValue('class', 2);
  }
  if (formid == "112") {
   nlapiSetFieldValue('class', 1);
  }
 
}
}