1. I went on a school field trip to Jeju Island.

2. I went on a school trip to Jeju Island.

3. I went on a study trip to Jeju Island.

4. I went on a study tour to Jeju Island.

5. I went on a study travel to Jeju Island.

6. I went on a field trip to Jeju Island.

(Which expressions are grammatical? Students go to places far off and they spend about 3 days traveling here and there to learn lots of things.)

#5 is a bit unusual, but the rest are all perfectly fine.

The answer is number 1.

All of the expressions you provided are grammatically correct. However, the suitability of each expression depends on the specific context and purpose of the trip. Here is a breakdown of each expression:

1. "I went on a school field trip to Jeju Island."
A field trip typically refers to a short educational trip taken by students outside of their regular classroom setting. It usually involves visiting a specific location to gain practical experience or hands-on learning. This expression suggests that the trip to Jeju Island was organized by the school for educational purposes.

2. "I went on a school trip to Jeju Island."
This expression is similar to the first one but without specifically mentioning it as a field trip. It implies that the trip was organized by the school, but the exact purpose or activities of the trip may not be explicitly stated.

3. "I went on a study trip to Jeju Island."
A study trip emphasizes the educational aspect of the journey. It suggests that the primary purpose of the trip was to engage in academic or research-related activities. This expression indicates a focused and structured learning experience.

4. "I went on a study tour to Jeju Island."
A study tour is similar to a study trip, but it often involves visiting multiple locations or institutions to study a particular subject or topic. This expression implies that the trip was more extensive and comprehensive in terms of academic exploration.

5. "I went on a study travel to Jeju Island."
While not incorrect, "study travel" is not a commonly used term. It may be interpreted as a combination of study and leisure travel, suggesting that the trip involved both educational activities and sightseeing or recreational aspects.

6. "I went on a field trip to Jeju Island."
Similar to the first expression, this statement explicitly highlights the trip as a field trip. It emphasizes the experiential learning aspect and suggests that the main objective of the trip was to provide students with practical learning outside the traditional classroom environment.

Overall, the choice of expression depends on the specific nature and goals of the trip, as well as the preferences of the person describing it.