You can use the index of the current item to check whether it is the 5th or more item. The each() callback gets the index of the current item and the current item as 2 arguments.
//Default value of the coupon is $200
coupon_value = 200;
//If there are 5 students or more the coupon value is $300
teir_value = 300;
//For each coupon_value set its html to the coupon value variable
$('.coupon_value').each(function (i) {
$(this).html('$' + (i > 3 ? teir_value : coupon_value));
});
Demo: Fiddle
As you have used, if you want to refer to the current item in the loop you can use this
inside the each handler.