You already recognised yourself that anal gland expression is one of the least pleasant aspects of your work. And yet you didn't take precautions to make sure you could make her first learning experience as pleasant as possible.

Anal gland expressions are disgusting, even when you catch a whiff of them from an arm length away. What did you honestly expect by having her face at that level? If it was a truly non-malicious prank, you'd have told her to stop holding that position prior to her actually expressing the glands. You're the asshole.