To find out how many 2-liter bottles of soda Ms. Huber should buy, we start by calculating the total amount of soda needed for her 24 students.
Each student is estimated to drink 250 milliliters. Therefore, the total amount of soda needed is:
\[ \text{Total soda needed (in milliliters)} = \text{Number of students} \times \text{Amount per student} \] \[ \text{Total soda needed} = 24 \times 250 = 6000 \text{ milliliters} \]
Next, we need to convert milliliters to liters, since each bottle holds 2 liters:
\[ \text{Total soda needed (in liters)} = \frac{6000 \text{ milliliters}}{1000 \text{ milliliters/liter}} = 6 \text{ liters} \]
Now we need to determine how many 2-liter bottles are required:
\[ \text{Number of bottles} = \frac{6 \text{ liters}}{2 \text{ liters/bottle}} = 3 \]
Thus, Ms. Huber should buy:
\[ \boxed{3} \] bottles of soda.