validate text box

I have 2 text boxes. 1 called school name , another gpa. If school name is not empty I need to make gpa field as mandatory, if it is empty then optional . I wrote some Javascript together with Ajax.

function validSchool(obj){
            
            if (obj.value != ''){
                  var collegeGpa= new Spry.Widget.ValidationTextField("college2gpa", {validateOn: ["onblur","change"]});
                  
            collegeGpa.reset();
            collegeGpa.validate();
            
            
            }
      }
My HTml is here
<!---School 2 name--->
                                           <div class="aaprowindent">
                    <div id="college2" >
                  <span class="label"><div class="college2">School Name:</span></div>
                  <input name="college2" type="text" id="college2" size="40" onblur="validSchool (this.value)"/>
                  </div>
            </div>

   <div class="aaprowindent">
                    <div id="college2gpa" >
                  <span class="label"><div class="college2gpa">GPA:</div></span>
                  <input name="college2gpa" type="text" id="college2gpa" size="40"/>
                  <span class="textfieldRequiredMsg">Please enter a valid GPA.</span>      </div>
            </div>
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