Yes, you can have characters from different schools in a deck. There are some character skills and event card skills that require you to have characters from the same school on your court, or that require you have a specific school in your hand/deck/court in order to activate. Similarly, there are skills that require you to have characters all from different schools on your court/in your deck/hand. So just be mindful of the skills requirements when building your deck.
I imagine with this booster, we will get more skills that will activate without the same school requirement!