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I was re-reading the books and came across these passages:
Once, Aunt Petunia, tired of Harry coming back from the barber’s looking as though he hadn’t been at all, had taken a pair of kitchen scissors and cut his hair so short he was almost bald except for his fringe, which she left ‘to hide that horrible scar’.... Next morning, however, he had got up to find his hair exactly as it had been before Aunt Petunia had sheared it off.
Another time, Aunt Petunia had been trying to force him into a revolting old jumper of Dudley’s (brown with orange bobbles). The harder she tried to pull it over his head, the smaller it seemed to become, until finally it might have fitted a glove puppet, but certainly wouldn’t fit Harry.
On the other hand, he’d got into terrible trouble for being found on the roof of the school kitchens. Dudley’s gang had been chasing him as usual when, as much to Harry’s surprise as anyone else’s, there he was sitting on the chimney.
Harry sat up and gasped; the glass front of the boa constrictor’s tank had vanished.
So Harry's done Apparation, some vanishing spell, and transfiguration (with the old jumper) accidentally before he ever went to Hogwarts. Why doesn't accidental magic seem to happen around Hogwarts?