Admitting that we know next to nothing about this, I wondered if Class might be a pre-existing concept that was retrofitted into the Coal Hill School and Doctor Who. Maybe Ness had pitched this series as a standalone, and then someone said, "What if we made it part of Doctor Who?" That idea then opened up new possibilities.
That was my impression when I first heard of the concept. They retrofitted an existing concept to latch onto the DW audience. It sounds like the only connection to DW is school. I suppose there could be more that we don't know about, but that's how it seems at first glance.
Sadly, a pretty disappointing announcement. If they have to do a spin-off, use some actual DW characters!

Mr Awe