Here's what they used to do at my school( state school, Lithuania,90s): the more gifted ones had a few options: they could skip a year(i.e. from 8th to 10th grade), not follow the curriculum the rest of the class did and simply do something more advanced during that time. I'm not gifted but was probably smart enough to find school easy,but couldn't be asked to do too much,so teachers usually used to leave me alone.