The most logical theme for a story about a group of students on their first field trip would be "New experiences are of great value."
This theme encompasses the excitement and growth that come with trying something new, which is a central aspect of a field trip. While the other options touch on elements of the experience—such as the excitement and potential fears—focusing on the value of new experiences captures a broader and more positive perspective that highlights the significance of the field trip in the students' lives.