There is no specific number. Under Indiana Law, the maximum number of students that can be transported in a school bus corresponds to the seating capacity designated by the manufacturer of the bus. Thus, a 72-passenger bus can carry 72 students, regardless of their age or size. Federal regulations govern how manufacturers can determine seating capacity, using a 15-inch block for each designated seating position and rounding up to the nearest whole number. Most school bus seats are 39 inches wide; dividing 39 by 15 produces 2.6, which rounds up to 3 seating positions per seat.
Clearly, that formula is not appropriate for all students. While state law does not limit the number of students per seat, it does require that aisles and exits be free of obstruction. This means that students cannot be hanging off the seats into the aisles, and their belongings cannot block emergency exits. A further consideration is that the passive restraint system called compartmentalization works only for students who are completely contained within the seating system; a student who is partially off the seat is not fully protected. Therefore, the number of students that can safely sit on a school bus seat is the number that fits entirely on a seat.