Take the third portal (yellow).

You step through the yellow portal. There is a flash of yellow light and a sensation of falling...

You find yourself lying in the centre of a large room with whitewashed walls, lit only by candles. As you sit up, you see that you are sitting in the exact centre of a big pentagram painted onto the floor. You appear to be unharmed, and are wearing a shirt and shorts made of some rough cloth.

Along one wall of the room are a number of bookshelves and assorted junk. In one corner of the room are some wooden steps leading down to the next level. A trail of faintly glowing dust leads towards the steps. You look around at the walls and the ceiling, but there don't appear to be any other exits from this room. You can hear some muffled shouting coming from down below. Without warning, you hear a voice in your head: "Warning. Academy security breach. Please leave the Arcane Academy in an orderly manner."