var students = [];

​

function addStudent(student) {

  // Check if we already know about this student.

  var existingRecord = students.find(function (s) {

    return s.student_id === student.student_id;

  });

  var classInfo = {

    class_number: student.class_number,

    location: student.location

  };

  if (!existingRecord) {

    // This is the first record for this student so we construct

    // the complete record and add it.

    students.push({

      student_id: student.student_id,

      classes: [classInfo]

    });

    return;

  }

  // Add to the existing student's classes.

  existingRecord.classes.push(classInfo);

}

​

addStudent({

    "student_id": "12345",

    "class_number": "abcd",

    "location": "below",

});

​

addStudent({

    "student_id": "12345",

    "class_number": "efgh",

    "location": "up",

});

​

addStudent({

    "student_id": "67890",

    "class_number": "abcd",

    "location": "below",

});

​

console.log(JSON.stringify(students, null, 2));