Book where teenage boy is able to "walk" to alternate realities

Question

Looking for a book I read in 2017. It was at a 8th-10th grade reading level, part of at least a two-book series (maybe more). Likely around 200-300 pages. The front cover looked like a person jumping between large balls, similar in appearance to disco balls. I don't remember the names of the characters, or the book title. The initial setting was similar to the present-day, the alternate reality academy was more futuristic.

The main character is a teenage boy. At one point, he suddenly finds himself unfamiliar with his surroundings. He goes back home, only to find his mother no longer recognizes him. His mother also has cybernetic prosthetics. He then runs into an armored man. The man takes him to another world. The main character learns that his ability is called "walking", a rare ability that allows him to travel to these alternate worlds. By imagining himself walking in his mind, the main character is able to travel. The main character then sees a creature resembling a bubble. He rescues it, but the armored man is fatally injured by a monster. The armored man is then revealed to be an older, alternate version of the main character.

The main character is then taken to a training academy, where he and other teenagers are training to fight the main villain. The headmaster of the academy has one bionic eye. One of the alternate versions of him had red wings. The main character makes some mistakes at the academy, and is expelled. He loses his memory of the incidents, and returns to his original ordinary life.

There, he learns that he has been missing for days. He and everyone around him is left puzzled by this. When a school bully attacks him, he brutally beats up the bully using advanced martial arts moves. Slowly, his memory starts to come back. When the main character watches his younger brother playing with bubbles, he recalls his pet, the bubble creature he rescued. He regains his memory, and goes back to the academy.

At the end of the story, the main character and his friends fight the villain, who was responsible for separating the souls of previous students and storing them. The students, with the help of the souls of the previous students, manage to trap the villain. However, the main character lets the villain go after the villain reveals he has trapped the bubble creature in a container. After the villain escapes, the container is revealed to be ice and melts, releasing the bubble creature. The students return to the academy, and vow to defeat the villain in the future.

In the end of the story, the main character goes back home to speak with his mother, who acknowledges that she may not see him again. She gives him her blessing, and he resumes his training with his friends.

Answer 1

Possibly InterWorld by Neil Gaiman and Michael Reaves,

... about Joey, a boy who finds out the hard way that he is a "Walker", meaning he can jump between the many alternate universes.

There is a winged alternate female Joey, and another is a cyborg.

The events of the book take a long time for him, but he's only gone from home for 36 hours (and has his memory erased when he returns). At the end he goes back to Walking knowing he'll probably never be able to see his family again without endangering them.

He befriends a multi-dimensional life form named Hue, which might look like a bubble? I didn't see a good description in the linked TV Tropes page.

And one of the covers certainly looks like it could be remembered as "a person jumping between large balls, similar in appearance to disco balls":

Answer 2

I see there's already an answer, but reading this question, I definitely thought of The Long Earth by Terry Pratchett and Stephen Baxter.

Its protagonist is a teenage boy who can walk between universes.

Answer 3

I thought of The World Between by Andre Norton, which is from a long time ago. But it is a rather less violent story.