Here is the sentence again:
Latoya tried to concentrate on Dr. Lemon's dull art history lecture, but
the darkened classroom and endless slide show soon had her dozing in her
seat, Latoya knew that Beverly would poke
her with a pencil if Dr. Lemon bothered to glance their way.
The comma between seat and Latoya creates a comma splice. At this spot, there are two complete sentences joined with a comma alone, making a major sentence error. The darkened classroom and endless slide show soon had her dozing in her seat is a complete sentence. Latoya knew ... begins the next complete sentence.
To fix this problem, you could put a period after seat.