<PERSON> - How could <PERSON> have compulsory classes at the same time as optional classes?
In Prisoner of Azkaban, <PERSON> is known to take all 12 subjects that are proposed to students (among which 7 compulsory classes, and 5 optional classes, which are : Arithmancy, Ancient Runes, Care of Magical Creatures, Divination, and Muggle Studies). We learn at the end of the book that she has been using a Time-Turner to get to all of her classes simultaneously. Up until that point, it all adds up.
What doesn't make sense, though, is that at several points during the book, we see her having compulsory classes at the same time as optional classes. For example, at around page 135 (depending on the edition), <PERSON> disappears after a Potion class (during which she has helped <PERSON> with his Shrinking Solution). :
"Five points from <PERSON> because the potion was all right ! Why didn't you lie, <PERSON> ? You should've said <PERSON> did it all by himself !"
<PERSON> didn't answer.
<PERSON> looked around.
Where is she ?"
Here, we can confidently assume that she turned back time in order to attend one of her optional classes. But that seems very unlikely, because it would mean that no other Gryffindor or Slytherin took that same optional subject. Indeed, students from the same Houses and same year always share their 7 compulsory classes (DADA, History of Magic, Astronomy, Potions, Charms, Transfiguration and Herbology), and sometimes share them with another House (for example, the Gryffindors share their Potions and Care of Magical Creatures courses with the Slytherins) - at least until year 6 (when they can drop core subjects). 738
Which is why a mandatory class being at the same time as an optional one means that only <PERSON> takes this optional class. Which could maybe be true, although it seems far-fetched.
However, we come across other examples like this one on the following pages. On the next page, which is page 136 of the same edition (the current Bloomsbury one), and after exiting the Potions class, <PERSON> splits her bag because it is overloaded with too many books - books of subjects she can't have today - <PERSON> assumes, because their only classes this day are Potions, in the morning, and Defence Against the Dark Arts, in the afternoon :
"Why are you carrying all these around with you ? <PERSON> asked her.
You know how many subjects I'm taking, said <PERSON> breathlessly. Couldn't hold these for me, could you ?
But - <PERSON> was turning over the books she had handed him, looking at the covers - you haven't got any of these subjects today. It's only Defence Against the Dark Arts this afternoon.
Oh, yes, said <PERSON> vaguely, but she packed all the books back into her bag all the same."
So there again, it seems she has another optional course (that neither <PERSON> nor <PERSON> take) at the same time as a compulsory / core course (Defence Against the Dark Arts). Which would mean that she is the only one of the <PERSON> 3rd years taking that subject (whichever it is, Muggle Studies, Arithmancy or Ancient Runes). Which would mean that, in total, she is the only Gryffindor to take 2 of the 3 subjects that she takes without <PERSON> and <PERSON>. A bit more unlikely.
Again, at page 313, she misses <PERSON> because - here we can only assume - she wanted to go to one of these 3 optional classes.
We know that she has Arithmancy and Muggle Studies on Monday, as we learn on pages 117 and 281 (respectively), and one of these 3 classes on Thursday (which is the day they have Potions in the morning and DADA in the afternoon) - just for reference.
And then, on page 335, we see <PERSON>'s time-table for her exams, which says :
"MONDAY
9 o'clock, Arithmancy
9 o'clock, Transfiguration
Lunch
1 o'clock, Charms
1 o'clock, Ancient Runes"
(We can note that this time-table doesn't reflect on their regular time-table because they have Care of Magical Creatures on Monday afternoon and that course isn't in her time-table here, however they do have Transfiguration in the morning). 738