This is a design circuit for alarm, but it can be used for school bell. This circuit is based on two IC. There are 555 timer IC and CD4017 IC. This is the figure of the circuit.
To ring this automatic school bell to start the first period, the peon needs to momentarily press switch S1. Thereafter, the bell sounds every 45 minutes to indicate the end of consecutive periods, except immediately after the fourth period, (IC2 and IC3) and AND gate CD4081 (IC4). Timer IC1 is wired as an astable multi vibrator, whose clock output pulses are fed to IC2. IC2 increases the time periods of IC1 (4.5 and 3 minutes) by ten times to provide a clock pulse to IC3 every 45 minutes or after 30 minutes, respectively. When the class periods are going on, the outputs of IC3 switch on transistors T1 and T2 via diodes D4 through D12.
Resistors R4 and R5 connected in series to the emitter of npn transistor T2 gate. When SCR1 is fired, it provides ground path to operate the circuit after resetting both decade counters IC2 and IC3. At the same time, LED1 glows to indicate that school bell is now active.