When you return to the school (around an hour into the game) you'll reach a scene where upon trying to go through the hole in the wall you'll be stopped by Ayase who asks you if you're planning to trash SEBEC (pictured below).


Your answer to this question determines whether you'll be doing the SEBEC Route or the Snow Queen Quest.

1. If you plan on doing the Snow Queen Quest choose "Not really."
2. Speak with the female student inside Class 2-4
3. Talk with Tsutomu inside of the Library on the 3F
4. Visit the Drama Club near St. Hermelin Gym 1F and speak with everyone inside of the room
5. Talk with everyone inside of the Student Council Room on the 3F
6. Speak with Principal Ooishi inside of the Principal's Office on the 1st Floor
7. Enter the St. Hermelin Gym and go through the door near the stage. This is the Gym Storage Room and you'll be automatically removed from this room unless you completed all of the above steps.

If you did all of the steps listed above then you will now be able to loot the Snow Queen Mask and begin the quest.