To do what you want to do you need to create custom workflows - that is exactly what it is designed for. It is not possible using the default permissions

Thanks Brian.
I will spend some time working out a way to use the stages / transitions. The workflow system rather looks like a way to process the different stages of publication. Yet, will it solve my earlier problems:

• will it allow preventing users from viewing the other users' (from the same group) unpublished items in the category which I defined for them?
• will it allow preventing users at one moment of the stages / transitions (not sure about the difference right now) from having access to MY unpublished articles?
  I'd like to get an idea before investing lots of time devising the whole thing.

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Hey Brian,
I tested the workflow and that's only sad news.
The worflow does not allow article viewing per user. Meaning that the worflow is just here to process the steps I described above step by step, which is not what I needed most in my case. I just have 1 stage: from the student creating his article to me publishing it. And Joomla does already offer the possibility to do it without the workflow system.

So, to make you understand what I did:

1. I created the workflow
2. I created 2 stages: the draft (created by the student, and allowing him/her to modify their article as long as they wish before they transition it to the super user) and the publication (made by me, the super user)
3. I created 1 transition: from draft to publication
4. I associated one category called student posts
5. The user group being allowed to create articles (thanks to a menu link on the front end) has the following permissions: everything is disallowed except for "create", "edit own", "edit state" and "transitions"

When testing - SAME issue:
Expected result
As the articles are pending for publication (by the administrator), users should not be able to view the unpublished articles submitted by the other user. So, user 1 should only be able to view his own article, and edit it until it is finally approved by the admin.
Actual result
User 1 can view user 2's article title, click on it, read it but not modify it.
User 2 can view user 1's article title, click on it, read it but not modify it.
Users 1 and 2 can view the administrator's unpublished articles (!!) in any categories. Of course they can't modify them but they should not have access to these articles at all and this is absolutely nonsensical to me. 😬
Eventually another problem is present: once the article has been published by the super user, the original author of the article should not be allowed to modify it anymore. Well, they can! 😣

SO, workflow is NOT the solution and I'm left stranded for something that should make sense for a CMS. It should be feasible.
So, either it is a bug and that has to be solved or it has to become the most awaited for feature. I do not want my users to be able to view other viewers'productions not until they are validated by me. Or else there might be a workaround? but this thread has been going on in the community for a few years now without any solutions provided for the moment. (last one being: https://forum.joomla.org/viewtopic.php?f=839&t=1011667). There are no extensions providing this feature at the moment, as they all rely on Joomla's native user permissions system.

I then really need your eye.

I guess Joomla is not suitable to your needs then

ok, that must be it!

What you need is a view level like view own. We don't have that. I am converting this into a discussion. Thats the right place to define what is needed in detail.