I've worked in six different primary schools in the last 18 months, one victorian, one ten years old and the rest built in the '60s and '70's. One had some skimmed walls in the offices, the rest of the building was painted facing block, one was entirely painted facing block, three were tape and joint and only the victorian was skimmed throughout with the exception of a new block which was tape and joint.
Kids or teachers sticking pictures our using magnetic numbers etc. will not cut down on the time before redecoration at all so there's no saving there. Blu tack dries out as does tape, but nothing stays up that long in schools. Most stuff lasts only a term, with some stuff up for the entire academic year.
As for creative activity, I haven't seen a school which doesn't have an IT suite and interactive whiteboards in each classroom for a long time. They use those for their creative activities, and the walls are for displays.
My wife's a head teacher in a primary school and all the schools in her consortium are moving away from displays and now have a mural of some description and big screen active displays.
I can't see many people wanting to pay to have a wall skimmed with magnetic plaster to make it magnetic, and very few having it done when skimming anyway. It's too costly for no added benefit other than an alternative way to do what they are already doing.
I may be wrong time will tell, but I doubt it.