When it comes out later this year, the most creative new set of Lego bricks won’t be available in stores or from any traditional retailer. The only place you’ll be able to find it is in a classroom.

I find your lack of faith disturbing.

Mattel Inc., already one of the largest toy makers in the world, wants a piece of the plastic building block business now dominated by Lego A/S. The El Segundo, Calif.-based company famously behind ...