WordNet
n. a bell rung to announce beginning or ending of class
Wikipedia
The ringing of a school bell is a signal that tells a school's students when it is time to go to class in the morning and when it is time to change classes during the day as well as when students are dismissed from school. Typically the first bell tells the students that it is time to report to class and the bell that occurs shortly after that means that the students are late. There may also be a warning bell between the first bell and the late bell.
In some schools it may take the form of a physical bell, usually electrically operated. In other schools it may be a tone, siren, electronic bell sound, a series of chimes, or music played over an intercom. Most schools where people who require sign language more than hearing go to use hand signals.
Usage examples of "school bell".
And yesterday it nuzzled itself into the crook of my arm and stayed there quite still time the school bell rang and I had to go.
By the time the school bell rang and I walked home, the news was moving across Zephyr like a crackling wildfire.
Bolting up, she felt her pulse hammering as she sat in her rumpled bed, trying to sort out why the school bell was ringing at home, and where the girls were on Saturday.
The pealing of the school bell rang out over the sleepy little town, and soon children appeared, coming up Congress, ready for the start of another day's education.
Today it embarrasses many teachers to be reminded that all sorts of values are transmitted to students, if not by their textbooks then by the informal curriculum--seating arrangements, the school bell, age segregation, social class distinctions, the authority of the teacher, the very fact that students are in a school instead of the community itself.
But even so, when the school bell rang at three o'clock these afternoons the passionate few--all girls--stayed behind to hear adorable Miss Galtzer read another chapter of Wuthering Heights.
I must eventually have fallen asleep, because all I remember after that is being woken by the school bell the following morning.