Q: "these are four students, one is writing." Does this mean that "A student is writing something."?
A: yes

Q: "Woman and man sit next to their green truck." Does this mean that "A woman and a man are sitting next to a truck."?
A: yes

Q: "A girl bending backwards holding her hat." Does this mean that "A woman is exercising with a hat"?
A: it is not possible to tell

Q: "Two girls are ice skating with each other." Does this mean that "Two girls are ice skating in a park."?
A:
it is not possible to tell