To find the number of school-supply kits they could put together in 30 minutes, we first determine the unit rate of kits per minute.
The rate is: \[ \frac{40 \text{ kits}}{8 \text{ minutes}} = 5 \text{ kits per minute} \]
Now, we calculate how many kits can be assembled in 30 minutes: \[ 5 \text{ kits per minute} \times 30 \text{ minutes} = 150 \text{ kits} \]
Thus, the answer is 150 school-supply kits.