She's underage, so drinking at all is an issue

• Your title is about lying to you, not about drinking.

• You only mention hungover twice in your unedited post and its because she lied to you and not about drinking. ("She then got hungover (again, she lied and said she had the flu) ")

• If you suspected underage drinking by a student, what does your ethics/code of conduct/professional expectations sayr you need to do? Yet, no where in your post addresses this, only that about lying. If you act on the drinking, you have to tell people she lied to you.

This is exactly what I mean by this weird justification to do weird stuff. Lying to you is more important than underage drinking.