Here is the passage again:
Ever since Andre peeked at Melissa's paper during
the biology exam. Guilt has consumed him. Even the blueberries floating
in his cereal bowl seem like the accusing eyes of Professor Gregory, his
biology instructor.
The first part is a subordinate clause fragment. This part has a subject, Andre, and a verb, peeked, but the subordinate conjunction since keeps it from making a complete thought.