English question

I think you are saying the combination of just and peaceful is necessary.

Both:
1 We believe that education is the key to building a more just world.
and
2 We believe that education is the key to building a more peaceful world

make sense.

However, "more" is not needed in either of these sentences or the original. Its presence presupposes that the world is just and peaceful, to some extent, and that education will improve justness and peacefulness in the world.

Now the world may be just and peaceful to some extent, but "more" invites a debate about how just and/or peaceful the world is now.

This may detract from the more central concept that education is the key to building a just and peaceful world.

Because the subject is the world. And the word "more" is an adjective for the adjectives "just" and "peaceful".

While it makes sense to have it refer to only "just world", it makes even more sense that it refers to "just+peaceful world." Not to mention, the creativity of the author brings up a picture of the latter.

To begin with, I would not call the current world just nor peaceful. It's a fantasy drawn in mind, a useless but worthwhile attempt to try and make the world even a little bit just and peaceful.

But if you are to describe such a world as a creative spirit, then it would definitely be a wish to have a more just and more peaceful world.

Just and peaceful both qualify world so more has to qualify the whole phrase.

It's a modifier... Just as double plus good is better than"good".... More just is better than plain old "just"

Neither just nor peaceful have superlatives. You are either just or you are not, there are no comparatives as there are with good, better, best for example.