I am not sure I understand what is being proposed as the alternative.
How is the gifted class different than a normal class, but still below the next year’s class?
Incidentally if you pull them out together with a small cohort, you are getting in the way of them learning how to deal with the rest of the society. And obviously once they get to college or a job, they haven’t learnt this crucial life skill.
Only for one or two classes. Not for the entire school day.
For this we have personal experience. It is not as bad as it is made out to be. My son had a lot of friends that were one or two years ahead. He still does have mixed aged students in his current classes in college. No one thinks twice about this. Incidentally you should put them in the upper level class with the advanced class; Not the slower kids in the upper level class.
Better still, the teacher can exclude them when curving the upper level class. So whoever is in the upper class will not be affected by these kids, and won’t feel insecure.
These are just details. I am saying once you pull the kid out of the entire system, you are significantly impairing his ability to learn core skills that are even more important to him/her than for the normal kid.