Polymorphism in Swift (if not)

I have this code in swift: To explain the idea of polymorphism!

//Polymorphism


class Person {
var name:String="Guest"
var age:Int=0

init(name:String) {
    self.name=name
    self.age=0
}
init(name:String, age:Int) {
    self.name=name
    self.age=age
}

func Update(name:String) {
    self.name=name
}
func Upgrade() {
    self.age++
}
}



class Student:Person
{
var average:Float=100
func IsOk()->Bool {
    return average > 80
}
init(name:String, average:Float) {
    super.init(name: name)
    self.average=average
}
}


class Teacher:Person {
var Salary:Float=2000
init(name:String, age:Int, Salary:Float){
    super.init(name: name, age: age)
    self.Salary=Salary

}
func GetNetSalary()->Float {
    return 0.8*self.Salary
}

override func Upgrade() {
    super.Upgrade()
    Salary*=1.1 // add 10% to salary
}
}



var t1:Teacher=Teacher(name: "Ahmed", age: 28, Salary: 3000)
var st1=Student(name:"Test", average: 70)

var p1:Person=Person(name: "abc")
var p2:Person=Student(name: "Student1", average: 100) //up casting
var p3:Person=Teacher(name: "Teacher", age: 40, Salary: 3008)


var arr=[t1, st1, p1, p2, p3] //array of persons and teachers and students

for instance in arr {
if instance is Student {println("This is student")}
if instance is Teacher {println("This is teacehr")}  
}

In the end in the for loop How could I put such a condition to see if an element in the array is only a Person? Because when I type:

if instance is Person {println("This is a Person")}

This gives me an error because this condition is always true!