I use Yes! as a marker word but it means end the behavior to the dog. The bridge is designed to tell that dog this is waht I want keep doing it. If she is getting up on 'good' she may think it is a mark and ends the behavior.

Maybe not say anything at all for now and just go in feed and remind her to stay.

If you are training at an obedience club they are designed for competition unless they have a family dog class. Many smaller clubs do not have family sog or manners classes.

I think you said there was not much offered near you so you are a little stuck if you want the social opportunity of a group class. You may just have to be firm in your goals for YOUR dog. Having a GSD, most of the class helpers may be reticent to try to take your leash but if they move to I would firmly tell them NO! And I think you will do that! Unless they were thinking the boy could not handle the large dog and they did not want an escalation they should have asked for permission first.