NullReferenceException

i really don't understand why this error come to me so much times x.x i could handle it 2-3 times, but now i can't handle it again.. although i have build the exact same code with the last one..

so my code is guiSchool3.js (This script attached to Main Camera on Scene "A")

 #pragma strict
 var script : guiSchool2;
 var choice : int;
 script = GetComponent(guiSchool2);
 choice = script.flag;
 
 var tekstur1 : Texture;
 function Start () {
     
         
 }
 function OnGUI(){
     
     if(choice==1){    
         GUI.DrawTexture(Rect(Screen.width*0.125,Screen.height*0.125,Screen.width*0.75,Screen.height*0.75),tekstur1);
     }
     else if (choice==2){
         
     }
 }

and here's guiSchool2! (i delete the part that not linked by the error)(This Script attached to Main Camera on Scene "B")

 #pragma strict
 
 var flag: int;
 
 function OnGUI (){
 
     GUI.DrawTexture(Rect(posbg_w,posbg_h,bg_w,bg_h),bg);
     if(GUI.Button(Rect(pos_b1w,pos_b1h,b_width,b_height)," ",button_perkalian)){
         flag = 1;
         Application.LoadLevel("School3");
     }
     if(GUI.Button(Rect(pos_b1w,pos_b2h,b_width,b_height)," ",button_luas)){
         flag = 2;
         Application.LoadLevel("School3");
     }
     if(GUI.Button(Rect(pos_b1w,pos_b3h,b_width,b_height)," ",button_satuan)){
         flag = 3;
         Application.LoadLevel("School3");
     }
 }

and the console throw me this exception

NullReferenceException: Object reference not set to an instance of an object guiSchool3.Main () (at Assets/Scripts/School/guiSchool3.js:5)

i believe i've done the exact same with the code before.. but it's still wrong.. why?