What is it called when you use the same letter for every word in a sentence?

A tautogram from wikipedia : A tautogram (Greek: tauto gramma, "same letter") is a text in which all words start with the same letter. Historically, tautograms were mostly poetical forms ([1]). The difference between a tautogram and alliteration is that tautograms are a written, visual phenomenon, whereas alliterations are a phonetic one. Most cases of alliteration are also tautograms, though certainly not all since different letters can frequently take on the same sound (e.g., circle segment or Catcher Ken). Similarly, most tautograms are also alliterations, although exceptions exist when using letters with multiple pronunciations (e.g., crazy child or pneumatic plate).

The bell is just a signal that the set time of the end of class is passing. the teacher lets you know when it's okay to leave.