How to change default desktop for all users

[Şerban S.]

[...] I like how you see things...
The idea of freedom is, sure, attractive and I like that in GNU too.
But too much freedom in a n high school might lead to unwanted development. [...]

Isn't that the very purpose of the idea of education? To create the sense of belonging? Of being something greater than the trivial physical body?
I recommend you a reading: Pyotr Demianovich Ouspensky -- "The Psychology of Man's Possible Evolution"
Maybe this would help you understand the difference between "army" and "school".

[...] I'd love to be able to trust students ... But if they have the littlest way to break things, they'll use it.
We've rebuild our classrooms last year and they look like they have been used since 5 years already... (lights broken, emergency signals destroyed... ) It might be the doing of just a few of them but it clearly shows that I can't trust them all with keeping their environment clean and clear...  [...]

What you were describing here, is an army barracks facility and has nothing to do with "school".
From that perspective, with this kind of thinking, it's extremely difficult to find a solution.

I'm aware of the fact that building a learning environment, is a task for a team. What you describe here though, is a battlefield, where the (alleged) Teachers, dispute their supremacy.
And the students, do the exact same thing: they apply what they are taught by their "teachers".
So, one question for you is: Are you sure you are in the right place for what you want to be and to do?
Being up for that kind of task, requires a high level of resilience. Look for Tom Bilyeu's teachings about building discipline and resilience.
The lack of discipline at the level of the teaching personnel, is obviously reflected in what the students think and do.
Want a proof?
Here it is:

[...] To set a big project with wallpaper designing and all these things, I already discussed it with teachers and none of them wants to take that kind of project, the dean thinks it as a good idea if we have teachers to take the lead but won't force teachers to do it if they don't want to.
So for the moment I'm out of move in that regard. Next year we'll have a new Art teacher so I'll try to bring her in that project.
[...]

[...] And for your arguments, Sir Serban, I totally agree !  [...]

Thank you very much! I'm glad at least we think the same. As for doing things... As I see it and said it above, you're in the wrong place.
The teachers are to be taught years before anything can be changed in this place. They are the real problem, not the students.

[...] School is intended to provide a safe environment to develop skills for everyone.
But the best way to do that is to be, financially, able to procure that environment.  [...]

You're very wrong about that.
Money, is a substitut for SKILLS. If you lack a skill, you need lots of money to buy it, one way or another. The form or "shape" if you will, is irrelevant.
A skilled person, does all what is required. There is a lot to say about that... Essentially, what this school lacks, is SKILLED TEACHERS.

[...] Environment that I try to make as secured as I can without cutting to much of their freedom, cause I want them to learn to be free internet citizens by themselves.
[...] Because school is meant for everyone  [...]

The real freedom, is inside. If you are free inside, this will reflect outside you.
That means the bible verse "As above, so below."
Thales, made a geometric version of this verse, called "The Resemblance Theorem" if you remember. It's a triangle related theorem.

So... Rephrasing my question for you:
Are you sure you are ready to teach the teachers?

Because this is the way to make things work. They live in a tiny-tiny box they call "my life" or "my career" or "my money" or whatever they see appropriate to call it, but is exactly this: a very small cage. And they want and pretend everybody to live in their cage.
It's nothing but sad. Very, very sad...

Best regards, Șerban.