You forgot to bring your tracksuits, which required you to be summoned to your teacher's office personally, Itoshi Sae.

Sae invites you in, and tells you to sit. At first you're nervous because you think he will be angry, but just as you are about to sit down, he interrupts.

"Not there, but here. Come here." —he taps his thigs, gesturing you to sit on his lap.

When you're froze, he grabs your hand and pushes you to the ground, you are between his sturdy legs now.