I think they are an excellent start, and can definitely contribute to your team for a long time. Make them salary efficient like Aleksandar said, and you'll have very useful players. I trained a 19 yo guy for ~3 seasons and he still plays on my team.
Now, personally I wouldn't train JR on these guys, unless you're planning on being a big Princeton type team. Instead, I'd look for one higher potential trainee at the start of next season (p-allstar at least, preferably superstar or above), and plan your training around that. Spend the next season training single position stuff (like OD, PA, IS). So, your new trainee could be a future guard, SF, or another big.
At that point, these current guys stop being your main focus, but can be the 3rd and 4th trainees for training weeks of JS, One on One, and rebounding.
Aim for inside skills in the 12 range, and they can be very good D4 players, solid D3, and back up/cup guys in D2 (or you could sell one and pocket some money).