It's not great but mobile classrooms are very good when you need to expand but don't have enough demand for classrooms to fill an expansion of say 5-10 classrooms.
Obviously you want to build more than one at a time but building more than you need is not budget friendly. So aiming for 3 mobile classrooms and starting a 5y construction immediately makes a lot of sense.
Keeping these things permanently is just weird.
It depends on where you live. Building in the Netherlands is a compliance hell hole and you end up with a very large portion of the cost being getting permits, doing environmental assessments etc.
In the UK you have to go through a local council that will refuse pretty much everything you want on top of the Netherlands issues.
This is pretty much the reason why builders are focusing on high value and luxury apartments since a bigger project benefits from scale on the permitting side.