Hogwarts students are required to have three sets of plain black work robes. All pupils’ clothes should carry name tags (PS5).
Mrs. Weasley took Ginny to a second hand robe shop in Diagon Alley to buy robes for her first year at Hogwarts (CS4).
Hogwarts robes are supposed to be long, covering at least part of the ankle (HBP6).
Harry and Ron both needed new school robes from Madam Malkin's for the beginning of their sixth year because they had grown so much over the summer (HBP6).
Percy Weasley gets new school robes when he becomes a prefect (PS6).