I actually have one of those Simplex school clocks; it had a clutch/solenoid mechanism which was operated via the master clock (don't remember how on that particular one). I took the 'slave' innards out and, yes, it CAN be easier to wait for the proper time to plug the clock in rather than removing the spring for the glass 'cause getting that spring to cooperate is a REAL PAIN!!!
I've also seen an IBM school clock circa 1953 or so that had a small vacuum-tube receiver which apparently listened for a 1 KHz tone on the power feed. The clutch would engage when the tone was detected.
Just my two cents' worth...
