The rule further states that no more than 300 minutes of class time can be used toward the extracurricular component (i.e. marching band) within the school day.

So, if a band class is single-blocked at 90 minutes, they will be fine to rehearse the entire period.

If a band class is double-blocked at 90 minutes, there must be other components involved for the other 150 minutes of the week.