+1
Can anyone tell me what the difference of the 3 words are? And the proper use of each.
Please leave 12 students' desks in the classroom. This sounds like you mean the desks of 12 specific students eg Tom's desk, Mary's desk, Bob's desk, etc.
Please leave 12 student desks in the classroom. This sounds like you just want 12 desks.
Comments 
Hi,

Can anyone tell me what the difference of the 3 words are? And the proper use of each.

Consider these examples.

one student

100 students

The new student's name is Mary.

Do you need more explanation?

Clive
what about students'

What is the difference?
Hi Anon

Students' is the possessive form of the plural noun students:

- The new students' names are Mary and Ellen.
Please leave 12 students' desks in the class.