Problem returning a collection from a web service

public class Schools : CollectionBase
{
   public School this[int index]
   {
      get{return ((School)List[index]);}
      set{List[index] = value;}
   }

   public int Add(School value)
   {
      return (List.Add(value));
   }

   public int IndexOf(School value)
   {
      return (List.IndexOf(value));
   }

   public void Insert(int index, School value)
   {
      List.Insert(index, value);
   }

   public void Remove(School value)
   {
      List.Remove(value);
   }

   public bool Contains(School value)
   {
      return (List.Contains(value));
   }

   protected override void OnInsert(int index, Object value)
   {
      // Insert additional code to be run only when inserting values.
   }

   protected override void OnRemove(int index, Object value)
   {
      // Insert additional code to be run only when removing values.
   }

   protected override void OnSet(int index, Object oldValue, Object newValue)
   {
      // Insert additional code to be run only when setting values.
   }

   protected override void OnValidate(Object value)
   {
      if (value.GetType() != typeof(School))
         throw new ArgumentException("value must be of type School.", "value");
}
}

and here is the code written in my webservice

[WebMethod]
public School GetSchoolList()
{
   School school = new School();
   school.SchoolName = "School " ;
   school.Address = "Address " ;
   return school;
}