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 seat makes a fragment. As all eyes in the room darted from her to Professor Williams is a subordinate clause, which cannot stand alone as a complete sentence.
You might want to review the rules.