From the author:  World of Solaria Reading Order:
Here’s a quick list of two reading orders for the crossover series. A lot of readers enjoy publication order so that they get the full Easter egg/crossover experience. Others prefer Timeline order: Ruthless Boys, Zodiac Academy, Darkmore Penitentiary. Still, others prefer to read series by series with Zodiac Academy being the first in the world of Solaria they discovered. There’s no right or wrong way to read!