What's the Difference between "all" and "all the"

Question

Can I leave out the word "the" from the following sentences? What's the difference between "all" and "all the" in the sentences? Are they all correct without the word "the"

1. I read in class 12 at xxx school. All (the) students in my school are very disciplined.

2. Oxford is the best of all (the) universities in the world.

3. All (the) schools in America are very rich.

4. When the teacher came to our class. All (the) students in our class were present there.

5. The giraffe is the tallest of all (the) animals in the world.

Answer 1

All schools in America are very rich.

We're making a standalone statement about schools.

All the schools in America are very rich.

For some reason, schools is distinctive. A typical reason we'd want to be distinctive is if the listener needs to know schools are rich, versus other things.

Here's some possible reasons for distinctiveness that could apply to the other examples. These are only examples to illustrate. Only the specific context established by previous conversation could tell the exact reason. If the statement doesn't exist in any context omitting the is best.

All the students in my school are very disciplined (as opposed to students in other schools)

Oxford is the best of all the universities in the world (as opposed to other educational or other type of institutions)

When the teacher came to our class. All the students in our class were present there (possibly someone else was supposed to be there)

The giraffe is the tallest of all the animals in the world (as opposed to plants)

Answer 2

They are correct, but a bit more informal when you leave out "the".

To be precise, generally in the examples you have shown, rather than just "all", "all of" is actually implied, as in "All (of the) students..."

With these words omitted, they are simply implied, resulting in a more "verbal" tone.