Did Amanda already have an inkling on who Preston was?
When "Reminiscing Guy" is bothering Preston, Amanda is sitting nearby and she starts smiling when learning about an embarrassing story about Preston.
https://www.youtube.com/watch?v=Il-gKZyMJ-Q
She may have at the very least, gotten a small glance or glimpse of Preston. So we can't just say that she absolutely has no idea what he looks like before it's pointed out to her in the yearbook.