THE MASK OF THE KINGFISH is a 46,000 word middle grade fantasy novel.
He who has the masks has the power. I don't love your hook...it's rather vague and doesn't tell me much about your character or story. But that's just my opinion.
When 12-year-old Wally Bennix gets arrested for starting an illegal garden to help the oppressed Shadow People, wait, what? That leaves me reeling.
Maybe introduce it a little slower. Set up who he is, what his problem is before he gets arrested. And if you aren't going to mention the Shadow People again, don't bring them into this. It's just confusing. he’s given two options: go to prison, or become an assassin. Never one for cramped spaces, Wally heads to the Assassin Academy.
While training for a mission to take out Avarcubus, the power hungry leader of their rival conglomerate (conglomerate of what? Corporations? what kind? What does that have to do with an academy? How are they rivals?), Wally finds the mask (explain this succinctly. When you really pare down the story, it's not really that important) owned by the Kingfish, his conglomerate’s leader. The mask grants Wally superhuman strength and speed.
There are three other masks owned by the three leaders of the other conglomerates that rule the world. Each mask grants unique godlike powers. As long as no one puts on their mask, the world is at peace. But now that Avarcubus is wearing his mask, he sends assassins to murder Wally’s leader, the Kingfish. Wally must travel to the other conglomerates to steal their masks before Avarcubus steals them first. The freedom of his people depends on a lowly thief.
^ Too much backstory. All we really need to know are your character, his problem, and what happens if he fails.
I know pretty much nothing about your character. Except that he did something illegal(though I can't say why gardening is illegal, unless he's growing weed, which is kind of the impression I get from that but I hope it's not what you mean 'cause this is MG) and became an assassin at age twelve.
I don't really know what his problem is. Something about his people? But you never really say what's happened to them.
I know what happens if he fails (his people don't get their freedom?) but I don't really care because I don't know enough about the problem.
Anyway, I hope that helps a bit. You've got a good start, and it sounds like a fantastic read, you just need a little more direction in your query. I'd definitely focus a lot more on your protagonist.
Best of luck with revisions!
(oh and please check out my query if ya don't mind :) )