Anyone can reuse diagrams or figures for unpublished material without requesting permission however there must be proper citation and referencing to give credit to the author.
DALRO advises that the copyright law states that “If a student's usage for images/figures falls into the category of academic study, review or critique, it falls within the ‘general exceptions from protection of literary works,’ which allows for the exploitation of literary works for the purposes of ‘research or private study by, or the personal or private use of, the person using the work’ and for the purposes of ‘criticism or review of that work or of another work’ (section 12(1)(a)(b) of the Copyright Act No. 98 of 1978). This means that the work does not need to be licensed".
If students do not ask permissions what happens?
This is an infringement of copyright. Meaning, a 'substantial portion' of work has been copied without the Copyright owner's permission. You will not be able to complete your course as you will not submit should your supervisor pick this up.
If your work gets published without anyone picking up the infringement, you may get a copyright infringement notice which may request you to take down the work and may also have a penalty fine. If you fail to respond to a notice, you or the university / publisher may be sued.
How to apply for copyright clearance?
You may contact the library copyright officer for assistance or you can use these steps of how to get permission.
The fair use / dealing copyright section allows a user to copy, for their own study or research or private use, as much of the work as they need to meet their reasonable needs, without seeking permission from the copyright owner or paying compensation.
The South African copyright law doesn't specify how much you may copy within the bounds of fair use / dealing, but it is clear that it must only be for your own use. So, multiple copies are outside of fair dealing.