…make them always the one that comes back, upgraded.
Hero soldiers are those that people got from various events. They got 4 stars back when that wa a major bonus - now 4 stars is kind of ordinary for many players. And a medal - HotSU or MoH or Iron Criss, etc
But hero solkdiers cannot be sent to the academy at all 
They are kind of a Gimmick really, I use them in squads that I’m not too bothered about levelling up certain troops in.
Such as my SU Infantry Squad has my 4* Gunner, as he’s only there for support.
Otherwise, why not make it so that the premium squads can be taken up to 5 stars too?
I would support that - but it has already been suggested numerous times - this has not.
For early game, having a 4 star trooper is a benefit because you can add all the best perks, but like with all things, I don’t see why they should be allowed to become better?
The premium squad troops that have 4* already arguably have better weapons (in some cases) so having an additional perk would need some form of balancing.
I guess each to their own, but I think at the moment it is fine, as they were unique rewards