Should states/regions etc. have a right to secede from their nation-states?

[muon2]

Let's think about this from small to large in the context of a representative democracy.

Should part of a school district be able to secede from a larger district to form a new district?
That should depend on the management of debt and the ability to adequately educate all the children in both districts. It should require the consent of both districts and the state which governs both districts (by statute or judicial decree).

Should a city that overlaps multiple counties be able to secede from those counties and form its own independent entity?
This happens in VA with some regularity to form independent cities. Broomfield CO did this in 2001. Broomfield did so by means of a state constitutional amendment in 1998.

Should a portion of a state be able to secede to form a new state?
There is a formal mechanism to do this through the US Constitution. It requires the consent of the legislature involved and Congress.

Should a US state be allowed to secede?
SCOTUS said that the union is inseparable once made. Any reconsideration would require the consent of the states. Presumably there would have to be a constitutional amendment ratified by the states to permit it, not unlike Broomfield getting a state constitutional amendment to secede from its counties.

Can this logic be extended to other parts of the world?
The above examples were rooted in the assumptions of representative democracy. So, let's consider a European example where there are strong democratic institutions.

Should Catalonia be allowed to secede from Spain?
By extension, this should require the consent of Catalonia and Spain and if membership in the EU/Eurozone is involved the consent of the EU member states as well.

Obviously this chain of thought doesn't have the same application in a situation like South Sudan where democratic institutions were lacking.