Ivy, basically a "sober grad" or "dry grad" just means a school or community-sponsored event that the kids can attend (at which there will, obviously, be no alcohol). This gives the kids an option.
Without giving them an event to attend, they will create their own party events. Which - "old enough" doesn't enter into it - where there's a will, there's a way, and at 18, they find a lot of ways.
So it is nice when something is arranged for them, that is fun, celebratory, and "dry."