At the very end of Philosopher’s Stone we find the following passage:
”Thanks," said Harry, "I'll need something to look forward to." People jostled them as they moved forward toward the gateway back to the Muggle world. Some of them called:
”See you, Potter!"
”Still famous," said Ron, grinning at him.
”Not where I'm going, I promise you," said Harry.
Philosopher’s Stone - Chapter 17: The Man with Two Faces
This seems a bit odd, because Harry’s friends call him by his first name. His teachers often call him by his last name. Malfoy calls him by his last name. Ernie refers to him by his last name when pontificating about the alleged attack on Justin. But these are not people who would have been saying goodbye to Harry at King’s Cross.
So who would have said “See you, Potter!”?