Here's what I often do with study hall, and I've NEVER lost a player to grades.
I start players at 10 minutes of study hall when they come in as freshmen. If the guy does well on mid-terms (3.0 or above) I'll adjust down to 5 minutes, if he does okay I leave it, and if he's highlighted yellow in the report at any time, I increase whatever he's at by 5 minutes.
Once the guy is a sophomore, I adjust to zero and hope it works. Sometimes I'll have to bump a guy up to 5 again if he's yellow on the report.
Most of my guys are above 3.0 at all times, with a few in the high 2s. You may say what I do is a little overkill, but most of my players also max out their skills too, and as I said, I've NEVER lost a player to grades.