Students appear to keep their own money in amongst their personal belongings. The opportunities to spend it are relatively rare until later years.
He didn’t have any Muggle money, either. There was a little wizard
gold in the moneybag at the bottom of his trunk, but the rest of the
fortune his parents had left him was stored in a vault at Gringotts
Wizarding Bank in London. He’d never be able to drag his trunk all the
way to London. Unless …
Harry Potter and the Prisoner of Azkaban - The Knight Bus
Harry gave Ron whispered orders and passed him some gold from under
the Cloak. They left Zonko’s with their money bags considerably
lighter than they had been on entering, but their pockets bulging with
Dungbombs, Hiccough Sweets, Frog Spawn Soap and a Nose-Biting Teacup
Harry Potter and the Prisoner of Azkaban - Snape’s Grudge
Money can be sent by owl
A second, very small parcel contained a note.
We received your message and enclose your Christmas present. From Uncle Vernon and Aunt Petunia. Sellotaped to the note was a fifty-pence piece.
Harry Potter and the Philosopher's Stone - The Mirror of Erise
Apparently they receive pocket money. Also, they can earn extra by taking work from other students.
GALLONS OF GALLEONS!
Pocket money failing to keep pace with your outgoings? Like to earn a
little extra gold?
Contact Fred and George Weasley, Gryffindor common room, for simple,
part-time, virtually painless jobs. (We regret that all work is
undertaken at applicant’s own risk.)
Harry Potter and the Order of the Phoenix - Professor Umbridge