I did exactly the same as Tobus. Only that the first one I followed him so they opened the door to the office for me. I entered after him and the guard, and killed both.

The second one I waited till he was outside the classroom, closed the door, killed him and both guards in the corridor with headshots before they could rise the alarm. Then a student comes out the classroom --> headshot him and everyone inside the classroom to make sure the alarm is not going to raise.

Then just peacefully walk outside :D