Turn back and cross the bridge, then head down to the riverbank and swim across the narrow channel to reach the underside of the Academy.
While Shani is permitted to enter the Academy, Geralt will need to find another way in. Optional Objective: Talk to Professor Shakeslock
You can also head directly to the Temple of Lilvani. After speaking to Shani, decide whether or not to venture to the Oxenfurt Academy to speak with the Professor about Gaunter O'Dimm.
