If you said Group A, you’re correct!
In Group A, the individual scores are very similar to each other and to the average score. The class is also bigger, so you’re working with more information.
That means that this average score has a high level of precision. You’re pretty certain that your calculated average score accurately reflects the true average score.
Group B is small and the scores are very different.
That means that there’s very little precision to these numbers and a lot of uncertainty about whether the calculated group average reflects the true average for students in this school.