She's not his mother. Riley knows this, even though it's hard to feel it, sometimes. She takes care of them, growing up, making them stronger. Dr. Walsh is a brilliant psychologist and biologist and who knows what else- she's a polymath, a genius, and inventor. She could revolutionize the world, if only it wasn't so hard to get anything done with maleficaria roaming the earth, killing any children. They snuff out potential, she had said. That's how she settles on her job: finding promising magical kids whose parents won't be able to prepare them for the Scholomance (independents have such low rates of survival, she reminds him), and taking them in as her own. But she's not a mother- just a scientist.
Riley is one of five boys. She sends them in one by one. It's with the first one, Forrest, that she realizes the major problem- when you use potions to make a growing wizard strong, he runs into trouble if he can't make his own supply. Riley thinks that seems obvious, but it's probably hindsight bias; Dr. Walsh is smarter than he could ever be, and she didn't predict it. The second one, Graham, is trained as an alchemist, but that's just not his strength; he dies when he can't improve enough on the potions she made for him. Dr. Walsh is always perfecting the formula. With the third, Liz, Dr. Walsh trains her in everything that her talents will allow and her situation needs- languages, creative writing and alchemy, in her case, but even though she makes it to graduation, that's the last they hear of her.
He's got a younger brother, left, Adam. Dr. Walsh is going to send him in next year, right after him. Riley's job: work on improving the potions on the inside, while Dr. Walsh and Adam work on that outside. When Adam comes in with their updates, they can compare and let the cream of the crop rise to the top. Riley wants his formula to win, of course, but it's not about that. It's about him making sure that the both of them make it to graduation. On the day of, Riley has everything he'll need: one of each potion to start, some reagents, a mana crystal, and a kiss on the forehead from Dr. Walsh. And then he's gone.