I am looking for a word that expresses the idea of a prison being difficult to escape because of its physical construction, in a similar way that ‘fortified’ means a structure is difficult to attack from outside because of its physical construction.
The context of this is something like, “The school building is more heavily guarded/fortified/etc. than the prison,” but without the sense that there are actually people there guarding it or that existing architectural measures (like barricades, fence gates, security balcony with bulletproof shields at intervals, etc) are in active use. Rather, it is just constructed in a way that would make it easy to guard, or has such features.
unable to be broken into or taken by force: an impregnable castle
While this suggests being attacked from without, the example you give, a school building, is usually concerned with intruders rather than inmates.
You also could consider inescapable
incapable of being escaped or avoided
This is probably more often used figuratively, referring to ideas or conclusions, but may be useful.