The rapid clicking of slave clocks in schools was a design feature of IBM (and later Simplex) clocks. The master clock sent out minute pulses to the slave clocks for the first 50 minutes of each hour on the "A" side of the circuit. Slave clocks operating on-time would automatically switch to the "B" side of the circuit and wait for the 51st minute pulse on the "B" side. Slave clocks operating slowly would remain on the A side and receive pulses at a rate of 1 every 2 seconds until they automatically switched to the "B" side at 50 minutes past the hour. Slave clocks that were running fast arrived at 50 minutes past the hour prior to the master and switched to the "B" and waited for the next pulse on the "B" side at 51 minutes past the hour. On the hour all the slave clocks switched back to receive the hour pulse on the "A" side. This was all accomplished by a cam in each of the slave clocks that moved a switch from the "A" side to the "B" side. For power outages (the master clock coul d continue to operate on a spring or weights but could not send out the impulses), the master clock could be fitted with an optional accumulator that counted the missed pulses. When power was restored, the accumulator would "unwind", sending out rapid pulses until the slaves could catch up. This could also be used to move the clocks ahead 23 hours (retard by 1 hour), or ahead hour for daylight saving time. Therefore the click-click-click in the classrooms as slow slave clocks caught up to the master.
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