D6L:
1. The weighted ball program that colleges do isn't designed for a 12 year old.
2. you can expect a 3-5mph gain from 12-14, easily, due to growth.
3. late fall/ early Winter is the time to work out HARD, and not throw a ball.
4. Long toss is what is recommended by most professionals. Possibly a weighted ball program later down the line, but all ages would improve with long toss.
5. I would guess over 25% of the kids get shut down from age 12-15. Too much throwing, plus growth plate issues....I saw a LOT of kids have to shut down and some that didn't but should have! Those are super tender years....throw with extra care.