There are no bright line rules. You can't just sit around and collect checks, you've got to seek other employment. You can't intentionally underemploy yourself to keep the checks big. Nor can you unreasonably decline a job because it's somehow "beneath you." You can't be expected to find a head coaching job that exceeds your current salary or is some arbitrary 75, 80% of it, either.
You have four years from the time the contract is breached until you must file suit or lose the claim. Without contract language to the contrary (setting out a "buyout" or "liquidated damages") you'd be entitled to the monthly or periodic payments you'd have been entitled to if still employed, offset by the income of whatever employment you obtain. So, your obligation to mitigate or avoid damages extends as long as your contract might have, or resolution occurs, by lawsuit, settlement, whatever.
In a trial, the coach is apt to argue that he couldn't get any better job than whatever he got, whatever it is, while the school is apt to argue that he should have sought/accepted some assistant/coordinator job at some large percentage of his salary. The jury gets to sort it out.
Yes, it is a situation that begs for settlement/compromise.