Percy Jackson and Harry Potter belong to Rick Rioardan and J.K. Rowling. I have only written the fanfic.
It is believed that all the wizards and witches are descendants of Greek/Roman Gods and Demigods, mainly Hecate, since she is the goddess of Witchcraft and Magic.
Also, Ravenclaws, though bear the symbol of the eagle(Zeus), are supposed to be descendants of Athena.
They live separately from other demigods because a long time ago their ancestors decided not to take part in the Torjan War and thus not on good terms with Gods and Demigods. They also have left their Greek Culture long behind... although the power of Magic still runs in their blood.
It's the year 1996, the Giant's war has just ended and has left all the demigods with lots of scars. Percy Jackson and his friends have gone through a lot and now need a change of scenery to heal. Gods have decided (yet again) that this is the best time for them to again forge a good relationship between wizards and demigods...and only The Gods and Chiron know about this history yet.
Parallel to this Harry Potter is now gonna be in his fifth year at Hogwarts...with lots of surprises he doesn't know about, because he is locked in his house on Dumbledore's orders and growing restless day by day...until he finally gets the chance to go to his Godfather's house. But Harry doesn't know that there is big news waiting for him, quite something he wouldn't have imagined ever.