Karak (Case Markers)

Seeing "se" in "Ram wrote with a pen" (Ram ne kalam se likha), Mohan thinks "se" always shows something separating away. His teacher says "se" means something different here. Who is right?

After learning "Ram wrote with a pen" ("se" = instrument), Nandini thinks "se" always shows the instrument. So in "a leaf fell from the tree" ("ped se patta gira") she also treats "se" as an instrument. Her brother says the meaning is different here. Who is right?

Seeing "ko" in "I called Ram" (maine Ram ko bulaaya), Kartik thinks "ko" always shows who benefits from an action. His sister says "ko" shows something different here. Who is right?

After learning "I called Ram" (object), Priya thinks in "I bought a book for Ram" too, "Ram" is the same direct target (object) of the action. Her friend says the relationship is different here. Who is right?