In my experience, lerneja ekskurso is the mainstream translation when a high school class is involved, as in mi iros en/al lerneja ekskurso or mi partoprenos lernejan ekskurson. Lerneja tendaro is also used for a high school camp.
For scientists and researchers, with or without their students, the best expression depends on the type of activity: I’ve heard both esplorvojaĝo and surloka esplorado for linguists but for, say, botanists or geologists, esplor-ekspedicio might be better, as it conveys the idea of wild, little known, possibly uncharted territory. Archaeologists embark upon fosad-kampanjoj, and so on.
As Oliver Mason pointed out in his comment, the particular meaning that “field” has here is not conveyed by kampo.