Answer:To find out how many pizzas Mrs. Smith should order, you need to divide the total number of slices of pizza needed by the number of slices in one pizza.
Since there are 1224 students in the grade 4 and 5 class and Mrs. Smith wants to give each student three slices of pizza, she will need 1224 students * 3 slices/student = <<1224*3=3672>>3672 slices of pizza.
If a medium pizza comes with eight slices, Mrs. Smith will need to order 3672 slices / 8 slices/pizza = <<3672/8=459>>459 medium pizzas.
Therefore, Mrs. Smith should order 459 medium pizzas for her pizza party.