There are several phrases that an Engpsh teacher might use to indicate the start or end of a class:
"Let's begin class."
"It's time to start."
"It's time for class to begin."
"It's time to end class."
"That concludes our class for today."
"See you next time."
"Have a good day/evening."
It's also common for teachers to use a bell or other signapng device to indicate the start or end of a class.