Basically, the game has a story. For the main characters, you experience the start and finish of their "Initial" route, and then you can choose whether to see the rest of it!
For side characters, you have to seek them out and activate their routes through previous choices, events, etc
At the end of the game, based on which routes you activated and which you didn't, you can choose who to "date". Basically you can activate every single route at the same time!
As for the Principal, sorry you will get the "Miel fan" treatment and will have to wait a bit T . T