Here is the original item:
When the cell phone began to chime in her book
bag,
(A) Jasmine
froze in her seat (B)
as all eyes in the room darted from her to Professor
Williams. (C)
An instructor who tolerated no interruptions during a lecture.
You wanted to do this:
When the cell phone began to chime in her book bag.
Jasmine froze in her seat as all eyes in the room darted from her
to Professor Williams. An instructor who tolerated no interruptions during
a lecture.
A period after bag makes a fragment. When the cell phone began to chime in her book bag is a subordinate clause, which cannot stand alone as a complete sentence.
You might want to review the rules.