When Sylv first sees Tyler in the doorway of the auditorium, who does she think it is and why?

This
question relates to This Is Where It Ends by Marieke Nijkamp.

It's become painfully obvious to Sylv that something's not right. Morning assembly's
over; the bell has rung; and yet, the doors to the auditorium have been locked. Even the
emergency exit has been locked. Something's clearly up, but Sylv doesn't know quite what it is
until she catches a glimpse of someone standing in the doorway to her left.

In the florescent light, she can make out a lone figure with blond locks that peek out
from underneath a black knit cap. The mop of unruly blond hair instantly reminds Sylv of her
twin brother. This is just the kind of stupid prank he'd playlocking all the doors to the
auditorium to prove himself the most interesting student at Opportunity High. But when the
figure in the doorway raises his arm, Sylv realizes that it's not her brother after all: it's
high school dropout Tyler Browne, and he's carrying a gun.