If this is across campuses, that's even more of a challenge.

In your shoes, I would focus on all of the MS requirements first, and fit in HS where possible. Assuming this was always a team teaching situation, continue going to the HS classes you were scheduled for. Focus on your MS sectionals after school.

I don't see where you have any additional obligation to the HS program. HS admin needs to hire someone PDQ. Plenty of December grads who would take an interim position. Maybe they can bring in a contractor to fill in part-time for the rest of the year.

If you want the head director job, maybe try harder to fit High School into your day, but you can't be in two places at once. Expecting you to prepare both ensembles for UIL contest without qualified help is completely unreasonable.