Sally-Anne Perks was a witch who started her first year of Hogwarts in 1991 (PS7).
Fans have speculated that Sally-Anne Perks left Hogwarts at some point before the end of Harry's fifth year. This is based on the fact that her name is not included by Flitwick when calling students in for their Charms practical O.W.L. exam:
The fifth-years ate lunch with the rest of the school (the four house tables had reappeared for the lunch hour), then they trooped off into the small chamber beside the Great Hall, where they were to wait until called for their practical examination. As small groups of students were called forwards in alphabetical order, those left behind muttered incantations and practised wand movements, occasionally poking each other in the back or eye by mistake.
Hermione's name was called. Trembling, she left the chamber with Anthony Goldstein, Gregory Goyle and Daphne Greengrass. Students who had already been tested did not return afterwards, so Harry and Ron had no idea how Hermione had done.
'She'll be fine, remember she got a hundred and twelve per cent on one of our Charms tests?' said Ron.
Ten minutes later, Professor Flitwick called, 'Parkinson, Pansy - Patil, Padma - Patil, Parvati - Potter, Harry.'
'Good luck,' said Ron quietly. Harry walked into the Great Hall, clutching his wand so tightly his hand shook (OP31).
If Sally-Anne were still at Hogwarts, her name would have been called after the Patil twins and before Harry.