I think that this has always been an option for students whose high schools allow for early graduation. The thing is, the student also has to be willing and interested in possibly graduating early and parents need to be OK with it. In the end, with all of those limitations, it is not that many people who would do it.
E.g., we offered our older daughter to possibly graduate early and apply to colleges, and she said no way; because she wanted to graduate with her friends. I still think she should have applied because she would have started thinking about what goes into a college application earlier (she is a procrastinator and she ended up doing her college essays right before they were due). My younger one is considering this. But, she has always been the more organized one anyway.