not a teacher
"who had flagged him up"
I haven't heard this phrase before but from the context I'm confident that you're correct. The scout in Portugal brought Bébé to the club's attention.
Bébé was in fact homeless as a young boy, having been abandoned by his parents. He was living in a shelter for the homeless when he was invited to play for a side in the 2009 European Street Football Festival. You can read more about his life here: http://en.wikipedia.org/wiki/Bébé
"a triallist" = a player who practises and perhaps plays some non-competition games with a club, but is not yet registered with that club. He is being trialled by the club to see if and how he fits in with their plans.