The story is quite old fashioned and What-Katy-Did-y.
If you compare Eustacia
you can see that EBD wants us to sympathise a bit with Eustacia - we get much more insight into the inner workings of her mind and moments where she realises her bahaviour is awful, as opposed to Thekla who is presented almost entirely from the 'outside', if that makes sense. Then her 'punishment/reform' seems very long because EBD devotes a book a term to Jo's head girlship and an extra one for the camp. Up until then the Tyrol books have been much more jumpy. But Eustacia's long recovery just gives Madge more time to shine which I suspect EBD was quite happy about. And more opportunities to ram her message home as well