Many of the players in the England U16s squad have, unsurprisingly, progressed from the England U15s. However the teams are run by different bodies.
The U15 team is run by the English Schools' FA, with players initially being put forward by their schools for inclusion in the district team (e.g. Sheffield U15), then being nominated from there for inclusion in their county team (e.g. South Yorkshire U15) and then nominated from there for inclusion in the national team.
The U16 team is run by the FA. It's theoretically totally separate to the England U15 team but you can be sure that the U16 coach (Kenny Swain) will keep a close eye on anyone who has been selected for England U15s, with a view to including them in his own side a year later.
So the 'unofficial' route into the England U16 team is through the ESFA U15 team. The official route is through a nomination from a Football Premiership/Championship/League club. Take a look at the current England U16 squad:
http://en.wikipedia.o...nder-16_football_team
Every player is signed to a professional club and it's that club which will have drawn Kenny Swain's attention to their talents (unless, as indicated above, he was already monitoring their progress through the ESFA teams).
Chris