Okay let me correct myself: I think this is a uniquely Western problem (or maybe it's a uniquely non-Middle Eastern problem Idk), because at least in Egypt this is not a thing, and in fact religious highschools (known as Azhar highschools) are notoriously hard because they teach you everything you'd be expected to learn from a typical highchool and on top of that the classical Arabic and other knowledge necessary for the student to pursue a degree in Islamic scholarship. This issue also seems like it's tied to a uniquely Western attitude towards religion, so I'm not sure how applicable it is to Islamic schools given that Muslims have a completely different idea of religion and spirituality.

They actually are exceptional. In many countries, religious schools are not for the purpose of avoiding laws of curriculums. Some countries force the curriculum anyways, and said schools can only add their own. In some places, there's not a hard rule to avoid.