Is there a former or current SCA guy out there with an experienced opinion?
Glasses are annoying at best under a helm.
Assuming, unlike most mammals, wearing a tin can over your head, holding in all that heat, and not exercising (like combat) you will not be sweating. Because, if like most mammals under those conditions, you will be sweating. Thus the glasses will slip.
To be honest, the REAL problem would be the tendency of glasses to fog up inside a full helm (or other confined space).
This is also a concern.
The biggest concern is that if you fall down, or get hit in the head, the glasses can get broken. Then you have glass in your eyes. which would cause alot long term damage. Penalty for wearing glasses under a helm, not much; penalty for getting glass in your eyes, blindness.
The SCA and most other groups, make the fighter remove glasses from under helms. Some are adjusted to the prescription sports goggles and allowing them. However they are made in theory for the rough and tumble of sports.
Most boffer groups that do not require protective head gear do not require removal of glasses. However, most of those groups do not allow head shots. In spite of not allowing head shot, they strong recommend you remove the galls, so they do not get broke.
I remove my glasses when engaging in assorted sports which are intended to replicate middle ages combat.