I just ran into the magic student. I think one of his spells must have hit Lydia because she took off after him and killed him. I was like, No! Lydia, Stop!

I am guessing the only way to get her to stop would be some type of calm spell. Would that work? I picked up Lydia when I was level 50 so she is maxed out. Would any type of calm spell work on her? Is there some other way besides illusion spells to get her to stop? I generally don't spend any perks in illusion and the only two spell I care about are clairvoyance and muffle (for training).

You want her to stop? Kill her.

Unfriend her! When she starts back to Whiterun, friend her again. It should reset her. You have to do it before she kills. She is your sword and your shield after all.

Use calm on the student?

She killed three Vigilants of Stendarr in my current playthrough. I tried walking away in the hope she would follow...which proved fruitless.

I guess that on top of having a dark side, she's a bit of a godless heathen, too. What are you going to do? Loot the bodies and sleep with one eye open...

Leave her at home.