...you don't understand what the Side-By-Side quoted from the TAC:

only the amount of time scheduled for that class may be used for practice purposes. (19 TAC §76.1001 [e] [1], [2] and [3].)

If they are scheduled in a class that meets in a 90 minute block, they are good to rehearse for 90 minutes.

As "Respectfully" asked, is there an ensemble on a 90 minute block out there that only rehearses for 60 minutes? While an interesting question, it is actually irrelevant...if the class is scheduled for 90 minutes, even if the director chooses to only use 60 of those minutes, the class continues for another 30. Therefore a band can go out and practice the full 90.

A band in that situation is following the rules...period. Now, there is an unused grip on the table...go get it.