Is this a good book plot?

A girl and her twin brother who live with a wierd old Broom-Maker in the futureistic olden days. (hard to explain) All the people of the town start going to school when they're 17, and the parent or guardien chooses their school. The broom-maker decides to send them so a school, where at first, seems like just a school for smart kids, but later turns into a shool, where they train evil scientists. The main character falls in love with a really evil dude, and he builds a time machine... I DON'T WANT TO GIVE ANYTHING ELSE AWAY...