What's under robes:
- PS: Snape lifted up his robes so Filch could tend to his "bloody and mangled" leg. No trousers.
- GoF: Before the Quidditch World Cup, Harry witnesses a wizard trying to convince another wizard to wear trousers (the wizard in question was wearing a flowery dress and insisting that he saw Muggles wear them). His indignant reply to the insistence on trousers was "I like a nice healthy breeze 'round my privates."
- GoF: When Harry wades into the water for the second Triwizard Task, he pulls off his shoes and socks. He does not take off any trousers, and there is no indication that there are trousers weighing him down as he swims.
- OotP: In Snape's Worst Memory, when James flips Severus upside down, we are shown skinny, pallid legs and greying underpants as the robes fall down around Snape's head.
- OotP: When Harry and Hermione follow Hagrid into the Forbidden Forest to see Grawp, Harry's arms and legs are covered in scratches after a short while walking, suggesting that they are bare under the robes.
- Dudley's tail pokes through his trousers. (Muggle)
- Harry wipes troll bogies off his wand using the troll's trousers. (Troll)
- Hagrid's sleeves and trouser legs are rolled up when he's shelling peas in PS. (Hagrid - not a wizard)
- Dudley hitches up his trousers when he finds Harry staring at Dobby's hedge in CoS. (Muggle)
- Uncle Vernon's trouser leg is in bloody tatters (dog-inflicted) after Harry blows up Aunt Marge in PoA. (Muggle)
- Cornelius Fudge wears a bottle-green suit (replete with trousers) when he meets Harry outside the Leaky Cauldron in PoA (Wizard in Muggle world).
- The aforementioned incident with the flowery dress in GoF (Wizard in Muggle world).
- Ron's pyjama trousers when he snarks at Harry about practising for his next interview in GoF (Wizard, at Hogwarts, but in pyjamas).
- Uncle Vernon hitches up his trousers as he interrogates Harry about the Dementors in OotP. (Muggle)
- Sirius mentions Kreacher snogging a pair of his father's old trousers in OotP. (Usage unclear, quite possibly Muggle trousers that Mr. Black had used when going into the Muggle world.)
- Mr. Weasley wears pinstriped trousers as he escorts Harry to the Ministry hearing in OotP. (Wizard in Muggle world)
- Lupin wears trousers as he meets Harry at King's Cross at the end of OotP. (Wizard in Muggle world)
- Harry wears jeans to Platform 9 3/4 in PS (Wizard in Muggle world)
- Harry's jeans are torn as he gets on the Knight Bus in PoA. (Wizard in Muggle world).
- Harry wears baggy hand-me-down jeans as he argues with Uncle Vernon at the beginning of GoF (Wizard in Muggle world).
- Arthur Weasley wears baggy jeans held up by a leather belt at the beginning of GoF (Wizard in Muggle world).
- Fred wears turned-up jeans at the Burrow right before they leave for the QWC in GoF. (Wizard headed into Muggle world).
- Mr. Weasley pulls on his jeans over his pyjamas after the Dark Mark is sighted after the QWC in GoF. (Wizard in Muggle world; they hadn't brought robes).
- Harry wears jeans and a sweatshirt before he boards the Hogwarts Express in GoF. (Wizard in Muggle world)
- Harry wears jeans at the Dursleys' and to his Ministry Hearing in OotP. (Wizard in Muggle world)
- Ron wears jeans at the Burrow before going to catch the Hogwarts Express in OotP. (Wizard going into Muggle world)
- Harry and the Weasleys change into jeans and sweatshirts before visiting St. Mungo's. Harry laments not wearing robes and thus being able to tread on Ron's foot when they encounter Neville there -- it's not possible to do so unnoticed without robes. (Wizards going into Muggle world)
- Tonks wears heavily patched jeans when she meets Harry at King's Cross at the end of OotP. (Witch in Muggle world)
- Harry wears a T-shirt under his Quidditch robes once.
- Draco claims that Ron's shirt is untucked in OotP, which makes me think JKR has either forgotten that they wear robes, or that Draco is being a git on purpose and taking points for non-existent shirts.
- No other shirts at Hogwarts but nightshirts and Weasley jumpers.
- Fudge has a "torn shirtsleeve" in the Dumbledore's office scene in OotP but he came disguised as a Muggle.
- Sweatshirts and T-shirts of all kinds are worn outside of Hogwarts (in the Muggle world or when about to enter it) by various people. One item of interest is a purple T-shirt worn by Tonks at the end of OotP -- it has a Weird Sisters logo, which has to mean that it was made in the wizarding world. The Weird Sisters themselves wear "artfully torn" robes.
- Robes are put on over their heads, which might explain the billowing -- the act of putting on robes is almost always referred to as "pulling them on". Wormtail pulls Voldemort's robes over his head when Voldemort rises again in GoF.
- It seems to be possible for robes to hitch up; Harry pulls them down at least once. It's possible to draw robes tightly around oneself.
- It's possible to pull robes up (ostensibly without revealing what's underneath) so apparently they're at least double-layered (this is also indicated by the fact that one can stuff things down the front of one's robes, cf. Hermione and stolen potion ingredients). Robes also have pockets, including inner pockets.
- Robe sleeves conceal bare arms (GoF, when Voldemort gives Wormtail his new hand, and when Snape shows Fudge the Dark Mark)
- Ron asks Hermione to leave the compartment so he and Harry can change in PS, yet in OotP they all change into their robes together (suggesting that they did not take off their Muggle clothes first).
- Robes have collars that can be turned up against the wind, going as high as the eyes (cf. Marietta). They do not appear to have hoods, however.
- Robes "hang" on people. It's possible to put on a robe back-to-front (so it seems like robes are similar to dresses).
- It's possible to grab a person by their robes to restrain their movement, even if said movement is very frantic (cf. Neville when Draco makes a remark about people whose brains were addled by magic; Ronan shaking Harry by the neck of his robes; Ron hanging off the front of Harry's robes in the Department of Mysteries). If robes were fastened by some normal means like buttons or strings; these would probably come undone under such force.
- It's possible to tread on the hem of your own robe and nearly trip yourself.
- Ludo Bagman's old Wimbourne Wasps robes are stretched tightly across his belly in GoF.
There are no references to how robes are fastened. They appear to simply go over your head like a dress. However, there is one reference of Ron having just one arm out of his robes in OotP.
- Fred and George wear jackets of "finest dragonskin" at the end of OotP. These are possibly a line produced for wizards who want to walk among Muggles in style.
- According to GoF, the Weasley children wear Muggle clothing during the holidays, but Mr. and Mrs. Weasley always wear robes.
- Madam Pomfrey wears a cardigan over her nightdress once in CoS, and Umbridge wears a cardigan (over robes) several times in OotP. Lupin does not wear a cardigan. :-P
snapesforte has a canon guide to Snape's clothing, with illustrations.
Conclusions regarding Hogwarts:
- There is absolutely no evidence or even mention of trousers worn by Hogwarts students. When boys prepare for bed, they take off their robes and put on their pyjamas.
- A T-shirt is worn once, and Weasley jumpers are worn at Christmas.
- The five points taken by Draco for Ron's untucked shirt in OotP may be either:
- Malfoy being completely unfair or
- A blunder on JKR's part, because that is the first and only time a student's shirt is mentioned. Knowing Draco, I'm leaning towards #1 myself. ;)