I recall (somewhat hazily, I admit) the sound of the school bell in high school. My memory says it was more of a buzzer sound than an actual bell. The fire alarm was more of a bell and that came in handy whenever someone called in a bomb threat out of fear that they might not pass the test in fourth period. But classes were regimented by the bell and it is no different here.
School starts at 7 with a study period allowing students to get prepared for the day. Then comes the first class at 8:00 AM. The first break is at 9:00 and then class resumes at 10:00 and on the day goes. At every one of these interludes we hear the call, the low baritone call of the two Ghanaian drums set up near one of the dormitories. Wub-buh-wub-bub goes the beat, or something approximate to that, for about five seconds. You can hear that across the entire campus without any issue at all. And when you are still wrapping up your long-winded lesson on why it is important to use 'Save As' before shutting down all the while the drums are rumbling, you will find that the students instinctively know you should stop talking and let them move to their next class.
While this superficially sounds archaic ("smoke signals wouldn't work?" is a comment which would not surprise me), it saves a heck of a lot of money stringing up alarms and bells all around the campus and making sure nothing shorts out or breaks. Just keep the drums in one place and beat them like there's no tomorrow at the appropriate time. Simplicity wins out, especially in a country where the power grid is on 95% of the time.