The answer to this question can be found at the beginning of chapter 7. Wade is in his final virtual class of the day. He is in Latin class on the virtual planet of Ludus, and the previous chapter ended with Wade telling readers that he believes the Copper Key is in the Tomb of Horrors. His mission now is to find the Tomb of Horrors.

As Latin class is trudging along, Wade is humming a song to keep his mind occupied while the teacher slowly conjugates Latin verbs. The teacher eventually gets to the verb "to learn," and she tells the class the Latin word is discere. Wade remembers that the clue has a part that says "much to learn." He also realizes that students are the people that have much to learn, and then, Wade realizes that "ludus" is Latin for school but could also mean "sport" or "game." That's when Wade realizes that the clue could be saying that the Tomb of Horrors is on planet Ludus, because Ludus is a school planet filled with people that have much to learn.

What group of people has "much to learn"?

Students. High-school students.

I was on a planet filled with students, all of whom had "much to learn."

What if the Limerick was saying that the tomb was hidden right here, on Ludus?

Once class is over, Wade scans the entire planet for something that resembles the Tomb of Horrors. He finds a match clear on the other side of the planet and proceeds to find a way to reach it.