GSON - Custom serializer in specific case

I have this schema :

public class Student {
       public String name;
       public School school;
}

public class School {
       public int id;
       public String name;
}
public class Data {
      public ArrayList<Student> students;
      public ArrayList<School> schools;
}

I would like to serialize the Data object with Gson, and get something like :

{ "students": [{ 
                 "name":"name1",
                 "school": "1"          //the id of the scool, not its entire Json
              }],
  "school": [{                        //the entire JSON
              "id" : "1",
              "name": "schoolName"
            }]
}

To make that, I must use custom serializer for the student part, so that Gson only print the id of the School. But for the School, I have to have nomal serializer.

How can I do everything with only one Gson object ?