Do eleverna bär mugglekläder medan de inte är i klasser på Hogwarts (som de gör i filmerna)?

Nej, i princip aldrig vid Hogwarts. I scenen i Fångaren av Azkaban som du har en bild av i frågan, till exempel, hade de kläder. Till exempel, efter att allt har hänt och de är i sjukhusvingen och Dumbledore antyder att de ska använda Time Turner, har vi:

But Hermione was fumbling with the neck of her robes, pulling from beneath them a very long, very fine gold chain.

Harry Potter and the Prisoner of Azkaban - p.288 - Bloomsbury - Chapter 21, Hermione's Secret

När de går till Hogsmeade på helgerna bär de påkläder. Detta var till exempel strax efter att Harry hade kommit tillbaka från Hogsmeade, när han hade kastat lera på Malfoy, Crabbe och Goyle och Malfoy hade sett sitt huvud i Hogsmeade när hans osynliga kappa slängde:

Harry followed him downstairs, trying to wipe his hands clean on the inside of his robes without Snape noticing.

Harry Potter and the Prisoner of Azkaban - p.208 - Bloomsbury - Chapter 14, Snape's Grudge

Vi ser också att de bär kläder under DA-möten, på kvällarna efter skolan (även om de är i uniform för DA-möten i filmerna):

'Well, my dad is very supporting of any anti-Ministry action!' said Luna Lovegood proudly from just behind Harry; evidently she had been eavesdropping on his conversation while Justin Finch-Fletchley attempted to disentangle himself from the robes that had flown up over his head.

Harry Potter and the Order of the Phoenix - p.350 - Bloomsbury - Chapter 18, Dumbledore's Army

Och under helgerna. De går till Grawp under den sista Quidditch matchen för säsongen, som är under den sista helgen i maj. När de går igenom skogen:

It was a great struggle to keep up with Hagrid, what with branches and thickets of thorn through which Hagrid marched as easily as if they were cobwebs, but which snagged Harry and Hermione's robes, frequently entangling them so severely that they had to stop for minutes at a time to free themselves.

Harry Potter and the Order of the Phoenix - p.606 - Bloomsbury - Chapter 30, Grawp

Även de bär kläder på jul. De tog Polyjuice potion på juldagen:

His shoulders stretched painfully and a prickling on his forehead told him that hair was creeping down towards his eyebrows; his robes ripped as his chest expanded like a barrel bursting its hoops; his feet were agony in shoes four sizes too small ...

Harry Potter and the Chamber of Secrets - p.162 - Bloomsbury - Chapter 12, The Polyjuice Potion

I The Philosopher's Stone hade de till och med klädselar för det mesta av hemresan på Hogwarts Express:

And suddenly, their wardrobes were empty, their trunks were packed, Neville's toad was found lurking in a corner of the toilets; notes were handed out to all students, warning them not to use magic over the holidays ('I always hope they'll forget to give us these,' said Fred Weasley sadly); Hagrid was there to take them down to the fleet of boats that sailed across the lake; they were boarding the Hogwarts Express; talking and laughing as the countryside became greener and tidier; eating Bertie Bott's Every-Flavour Beans as they sped past Muggle towns; pulling off their wizard robes and putting on jackets and coats; pulling into platform nine and three-quarters at King's Cross Station.

Harry Potter and the Philosopher's Stone - p.222 - Bloomsbury - Chapter 17, The Man with Two Faces

I själva verket har Harry brukade ha på sig kläder, vi får den här lilla sidan åt St Mungo, där de bar Muggle kläder för att blanda sig på resan till London:

(Harry wanted to stamp on Ron's foot, but that sort of thing is much harder to bring off unnoticed when you're wearing jeans rather than robes.)

Harry Potter and the Order of the Phoenix - p.454 - Bloomsbury - Chapter 23, Christmas on the Closed Ward

Det här är ett av mina husdjur som hatar filmen franchise, som det händer. Fördärvar nedsänkning.

De gör , men bär pyjamas i sängen.

Ja, ja de gör, åtminstone Weasleys.

Harry felt a slight sense of foreboding. He had rarely seen Mr or Mrs Weasley wearing anything that the Dursleys would call ‘normal’. Their children might don Muggle clothing during the holidays, but Mr and Mrs Weasley usually wore long robes in varying states of shabbiness. Harry wasn’t bothered about what the neighbours would think, but he was anxious about how rude the Dursleys might be to the Weasleys if they turned up looking like their worst idea of wizards. -Goblet of fire

Ännu Weasleys men på julen bär de jeans på tröjan / truppen som deras mamma gör dem

‘Hey, look – Harry’s got a Weasley jumper, too!’ Fred and George were wearing blue jumpers, one with a large yellow F on it, the other with a large yellow G.

They frog-marched Percy from the room, his arms pinned to his sides by his jumper.

Första gången, i filmerna, som vi ser att eleverna har ojämn kläder, i Hogwarts, finns den första filmen, Harry Potter och den Philosopher's Stone (2001) , även om det bara verkar vara under julhelgen, utanför lektionerna, under fritiden.

Enligt wikien är detta tydligen sant i böckerna, och de ger 3 exempel:

In the films, students wear normal Muggle clothing on their free time or outside of their lessons. This appears to be true in the books as well. Ron Weasley and his siblings typically receive sweaters from Mrs Weasley as Christmas presents, despite still having six more months of school left. In the sixth book, Draco Malfoy shops for new robes even though the school year is about to begin. Also in the sixth book, Lavender Brown asks Harry about Ron's - her boyfriend at the time - true opinion of her new dress robes.

Det är inte förrän den tredje delen av filmmanchisen, Harry Potter och Fångaren av Azkaban (2004) , som vi ser dem ha på sig ojämn kläder nästan hela tiden (utanför lektionerna, att äta i salen etc.), som du visade i ditt foto.

Detverkarvisserligenavvikafrånböckerna,därstudenternaärskyldigaattbärasinaenhetligakläder,som andra svar har visat.

Filminstruktören, Alfonso Cuarón , gjorde detta designval, medvetet från böckerna.

Alfonso Cuarón had the idea to have Harry, Ron, and Hermione wear everyday clothes more often than their Hogwarts uniforms in order to show more of the characters' personalities. He also gave the rest of the Hogwarts students permission to wear their uniforms any way they wanted to in order to bring a greater sense of realism to the wizards' school, hence why some of the students wear the uniforms very neatly while others have the shirts and ties hanging out.
- IMDB trivia (source)

I ett av intervjuerna i DVD-bonusfunktionerna talar Cuarón om hur barnen är äldre nu, boogeyman bor inte längre i sin garderob, den lever inuti dem, så han ville få en mycket naturalistisk känsla för barnens prestation och skrivning, och säger att de grovklara uniformerna och ojämn (muggle) kläder bidrog till att få den nivån av realism och individualism som han kände skulle lägga till den naturalism han ville lägga till.