Finding the School Kids, How many?

24 kids x 3 classes = 72 kids each grade

72 x 12= 864 kids in the school

The answer will be $24 \times 3 \times 12 \implies 24 \times 36 \implies (30-6)(30+6) \implies 900-36 \implies \boxed{864}$

If there are 24 kids in each class and there are 3 classes in each grade so each grade has $24*3$ kids which equals to 72 kids.

Now we know that there are 72 kids in each grade and there are 12 grades in a school so there are $72*12$ kids in school. Without doing the calculation wholly we know that the unit digit is 4. 864 is the only option which has the unit digit of 4 so 864 is the answer.