"All the students" and "all of the students" mean the same thing regardless of context.
When you qualify all three with "in the school", they become interchangeable. But without that qualifier, "all students" would refer to all students everywhere, and the other two would refer to some previously specified group of students.
An example of an exception: say a principal/headmaster makes an announcement addressed to "all students". Of course he means all students in the school—he's talking to the students under his authority and within the scope of whatever mechanism disseminates the announcement.