Final answer:
There are a total of 72 students in grades 2 through 4.
Step-by-step explanation:
In the second grade, there are 12 boys and 12 girls, for a total of 24 students in that grade.
In the third grade, there are 16 students.
In the fourth grade, there are twice as many students as in the third grade, so there are 2 * 16 = 32 students in that grade.
To find the total number of students in grades 2 through 4, we add the number of students in each grade: 24 + 16 + 32 = 72 students.