If 8 pupils can eat a carton of biscuits in 12 days, how long will it take 6 pupils to eat the same carton?

This represents the total amount of work (eating biscuits) done by one pupil over a certain number of days.

$$\text{Total pupil-days} = 8 \text{ pupils} \times 12 \text{ days} = 96 \text{ pupil-days}$$

Since the rate of eating is the same, the total pupil-days remains constant.

$$6 \text{ pupils} \times D_2 = 96 \text{ pupil-days}$$ $$D_2 = \frac{96 \text{ pupil-days}}{6 \text{ pupils}}$$ $$D_2 = 16 \text{ days}$$

It will take 6 pupils 16 days to eat the carton of biscuits.

The final answer is $\text{16 days}$.