What do you do?

Order the girls to detain the intruder

"Stop him," you yell, "don't let him get away!" The ladies gamely try to intercept, although they back off when he punches the smallest one in the face hard enough to knock her to the ground. Her companion kneels beside her, concerned, but the third runs up behind the man as he tries to escape and puts the toe of her stiletto into the back of his knee. He immediately collapses, screaming and clutching the joint, a stream of foul language pouring from his lips.

It takes a while to sort things out. First porters, then police, paramedics and university beadles are called. The man's knee turns out to be dislocated, but after all statements are taken, the student responsible isn't arrested and when it's discovered the lock on Dr. Renhouse's desk was picked and illegal lock picks are found on the perpetrator, he's the one who ends up in custody. The girl he punched is also taken to hospital, although the paramedics are confident she'll just have a black eye to show for her bravery. No doubt her assailant will have this added to his charge sheet in due course. You may not know the whole story for a while, possibly ever, but it's clear you've foiled some sort of crime.

After your own leg is examined and pronounced fine, just bruised, you text Natalia, your postgrad, to arrange your late lunch, walking over to the SCR, fortunately without pain.