JS - Closure Encapsulation

var school = (function() {
    var students = [];
    var teachers = [];

    function addStudent(name, grade) {
        students.push({
            name : name,
            grade : grade
        });

    }
    function addTeacher(name, speciality) {
        teachers.push({
            name : name,
            speciality : speciality
        });

    }
    function getTeachers(speciality) {
        return teachers.filter(function(t){
            return t.speciality === speciality
        });
    }
    function getStudents(grade) {
        return students.filter(function(s){
            return s.grade === grade
        });
    }

    return {
        as: addStudent,
        at: addTeacher,
        gs: getStudents,
        gt: getTeachers
    };
})();

school.as("ivan", 5);
school.as("ivanka", 6);

school.at("gogo", "maths");
school.at("gergana", "maths");

console.log(school.gs(5));
console.log(school.gt("maths"));