Affiliate Marketing Secrets

I just wanted to know how often this occurred before they started the Dominican Prospect League. I know that the A’s got Michael Ynoa back in 2008 but he went to an academy that wasnt affiliated with an MLB team.
I was wondering how rare it is that a prospect from the DR sign with a different team once he becomes eligible at 16.
And just as a side question if anyone knows a roundabout percentage of how many of these kids actually stay in school while they attend an academy that’d be great.
I know the Mets, Pirates, and Indians make sure of this but not so sure about others. Any knowledge would be appreciated