Making a connection between the Latin word ludus, meaning "school," "sport," or "game," Wade decides that the Tomb of Horrors must be on the school planet, Ludus.
It seems that Halliday was a big benefactor for the OASIS virtual school system, so to Wade, it makes sense that Halliday might stash the Copper Key there.
Plus, he might want a high school kid to find it. Who better to give a multi-billion dollar fortune to than a teenager?
After class, Wade scans the surface of the planet for a formation matching that of the Tomb of Horrors in the Dungeons and Dragon book.
He finds it! There's just one problem: it's clear across the planet, near Aech's school, and Wade's avatar has no method of transportation or teleportation. What, no bikes in OASIS?
Doing some quick thinking, he realizes that the school offers free teleportation to sporting events on the planet. His school is playing a virtual football game against OPS #0571, which is about an hour's run from the Tomb of Horrors.
Wade tells Aech he has homework, and teleports across the planet instead. In the forest, he finds the Tomb of Horrors and goes inside. Well, that was easy.