По поводу количества студентов. Вот, что написано на оф сайте
By J.K. Rowling
J.K. Rowling’s thoughts
Two of my most prized possessions are a pair of small notebooks, which contain my very first scribblings about Harry Potter. Much of what is written in them was never used in the series, although it is startling to come across the odd line of dialogue that subsequently made it, verbatim, to publication.
In one of the books is a list of forty names of students in Harry’s year (including Harry, Ron and Hermione), all allocated houses, with small symbols beside each name depicting each boy or girl’s parentage.
While I imagined that there would be considerably more than forty students in each year at Hogwarts, I thought that it would be useful to know a proportion of Harry’s classmates, and to have names at my fingertips when action was taking place around the school.
As the stories evolved, I changed the parentage of some of the original forty. While some never appeared in the books at all, I always knew that they were there; some had surgery to their names after their first creation; a few emerged from the background to have their own secondary stories (Ernie Macmillan, Hannah Abbott, Justin Finch-Fletchley), and one, Neville Longbottom, developed into a very important character. It is very strange to look at the list in this tiny notebook now, slightly water-stained by some forgotten mishap, and covered in light pencil scribblings (undoubtedly the work of my then infant daughter, Jessica), and to think that while I was writing these names, and refining them, and sorting them into houses, I had no clue where they were going to go (or where they were going to take me).
Here, then, are the original forty:
Abbott, Hannah
Bones, Susan
Boot, Trevor
Brocklehurst, Mandy
Brown, Lavender
Bulstrode, Millicent
Corner, Michael
Cornfoot, Stephen
Crabbe, Vincent
Davis, Tracey
Entwhistle, Kevin
Finch-Fletchley, Justin
Finnigan, Seamus
Goldstein, Anthony
Goyle, Gregory
Granger, Hermione – inserted in pencil, see crossed-out entry, below
Greengrass, Queenie
Hopkins, Wayne
Jones, Megan
Li, Sue
Longbottom, Neville – inserted in ink, see crossed out entry, below
MacDougal, Isobel [original name Katrina crossed out]
Macmillan, Ernest
Malfoy, Draco – inserted in ink, see crossed-out entry, below
Malone, Roger
Moon, Lily [first intimation of Luna Lovegood, this name was never used, but gave me an idea for a fey, dreamy girl. She was named before I decided on Harry’s mother’s name.]
Nott, Theodore
Parkinson, Pansy
Patel, Madhari
Patel, Mati
Perks, Sally-Anne
Potter, Harry
[Puckle, Hermione – crossed out, name changed and reinserted, above]
[Puff, Neville – crossed out, name changed and reinserted, above]
[Quirrel, crossed out, subsequently used for teacher]
Rivers, Oliver
Roper, Sophie
[Sidebottom, Neville crossed out]
Smith, Sally [Georgina crossed out]
[Spungen, changed to Spinks, Draco, all crossed out, re-inserted above]
Thomas, Gary
Turpin, Lisa
Weasley, Ronald
Zabini, Blaise
Но вопрос логичности всё-таки остаётся. 
