how can I delay generation of academy players ?

Currently my academy players are generated 2 days before the season end. I want to delay that into the next season.
I've tried upgrading my training facility but it didn't work.

Yes, each time you upgrate the timing starts over. But its gonna vary from season to season.

The time it takes to build an upgrade is the only delay.

You can't. But academy players are useless for the most part anyway.