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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...

Hi, I'm aware of European systems who run the seconds of a 'slave dial' on 220 Volts 50Hz, the seconds hand makes one full revolution in about 59 seconds and then stops till the Masterclock provides the 1 minute pulse @ 24 Volts DC, which releases the brake and the process starts all over again. This will keep the seconds hand in step with the master clock.
Minute pulses are positive , negative, pos. etc to prevent a clock jumping 2 minutes in case of contact bounce from the master.
In the drive from the synchronous motor to the seconds hand is a slip clutch provided which avoids stalling the motor.

Many railwaystations in The Netherlands and Germany use these types of clocks ( T&N and FAVAG ) The seconds hand makes a smooth pulsing movement and stops briefly at each second. "schleichenden sekunden".

In the Simplex system I think that a similar approch is used mains voltage for the motor and 24 Vdc for the minutes although not with changing polarity.