There are a couple of things you could try, but only one of them is safe to do in a classroom and is relatively unlikely to work.

What is happening is that in the water, the baking soda (sodium bicarbonate) is breaking up into sodium ion and bicarbonate ion. Meanwhile, the acetic acid in the vinegar is breaking up into hydrogen ion and acetate ion. The rate at which this happens depends on the strength of the dissociation; sodium bicarbonate is a soluble salt, which means that it dissociates very quickly. Acetic acid however is a weak acid, which means that (by chemical reaction standards) the breakup is much slower. Once the hydrogen ion from the acetic acid meets up with the bicarbonate ion from the baking soda, however, they combine to form carbonic acid. Carbonic acid then dissociates into carbon dioxide and water. Once there is too much carbon dioxide, it comes out of solution and becomes bubbles of carbon dioxide gas. This increase in volume from the transition from liquid to gas is what causes the explosion.

Thus there are three ways to slow this reaction down. They are:

1. Take the bicarbonate ion out of solution so that it cannot combine with the hydrogen ion.

2. Take the hydrogen ion out of solution so that it cannot combine with the bicarbonate ion.

3. Keep the carbon dioxide dissolved in the water and not become a gas.

(1) and (2) are difficult. For (1), you would need a metal ion that would grab onto bicarbonate. Aluminum would probably be your best bet, but I wouldn't want to play with aluminum ions because they're poisonous (and other ions, like lead, are worse). For (2), you would need a strong base to gobble up the hydrogen ions like sodium hydroxide. Unfortunately, that burns skin if it makes contact, so you don't want that either.

(3) is probably the easiest: dump ice chips into the water. Carbon dioxide, like most gasses, is more soluble in water at colder temperatures. By chilling the water to freezing by adding ice, you might keep less of the carbon dioxide from bubbling off. However, if there is still too much carbon dioxide, then it will still bubble off (and release heat as it does so, which will melt the ice).

If you add enough baking soda, it'll stop bubbling when it's neutralized the vinegar. I suggest measuring things beforehand so it's easier to change the direction of the reaction. If you could get a hold of some litmus paper, you could have two or three "stations" with more baking soda in one station, and more vinegar in the other, and maybe a third station that is plain fresh water (neutral pH), and ask students to test the pH of the solutions, before the reaction, predict what the pH (acidic or basic) will be after adding vinegar, then test the pH after the reaction. Litmus paper isn't always accurate, but kids love color changes!