It didn't specifically say who would be invited first. The implication was that all elders would eventually get to go.
My best guess is that it will work by seniority, as it did with the recent pioneer service schools that long-time pioneers could attend again. First those with 25+ years of pioneer service went, the following year those with 22+ years went, the following year 20+ years, etc.
Don't know if the "seniority" used for the elders' school would run by "time elapsed while serivng as an elder" or "time elapsed since baptism".