Why are frogs commonly used in laboratory dissections?
They are abundant, and easy to use
They are abundant, and you can find similar frog body plans no matter where you go. If you've dissected more complex organisms, you know that sometimes organs are sheltered, or hidden, which is great for the animal because it is harder to sustain damage but bad for you the dissector. You can split them, pin the flaps, and get an easy view. As amphibians, they also may be less of a source of distress for younger students.
It's mostly the ease of access thing. There are other small creatures available; like marine life. You could dissect a fish, but frogs are easier, and far easier to see the organs.