xylophone is probably the easiest to learn for your first instrument, you should be able to pick it up relatively quickly. it also has a similar layout to keyboard so if you don't like doing voice i would ask your teacher to change to keyboard.
coursework is pretty difficult, especially if you're crashing higher and have no prior musical knowledge. you will have to go back to national 4 and 5 concepts to progress forward.
i know you're prioritising sciences and putting most of your time into them, but if you do have some free time i would absolutely recommend looking through coursework and learning the concepts. i would suggest speaking to your teacher and see if there is any arrangements they can make to give you the best chance possible. best of luck to you !