Akid48=> I'm sorry regarding what you have to endure as a young person with parents in the BORG. (And you don't have to apologize for the length of your posting--If you look at many/various postings--including mine--they are very lengthy, and with/for good reasons.)
When it came to learning to play a musical instrument in middle school and high school, my sister, one of my closest cousins and myself had to sneak and take after-school classes because our programs/schedules did not originally accommodate this provision for us. Thank goodness the music teacher we had was very generous, in addition to the fact he said he had a distant cousin who was a JDub. He spoke as if he understood about the issue that the BORG had concerning extracurricular activities.
Now, I'm not trying to encourage you to become rebellious regarding your participation in the school band, but if it gets to the extent whereby you are not able to do so, I would encourage you to see if you can privately make an arrangement with your music teacher (or the band's faculty advisor) to allow you to continue practicing the tuba for a couple days after school. This is so that you will not lose your footing skill-wise with the instrument. Hopefully your instructor is an agreeable person, like mine back in the day was.
Furthermore, have you ever thought about learning an instrument of a small caliber, such as the guitar, violin, recorder (small flute), etc.? The reason why I'm asking is because I find, from my personal experience*, that these can be easily transported when you want to obtain lessons and still be able to enjoy playing music with these at your personal leisure. [*I was able to learn to play the piccolo and recorder, when I saw that it was difficult to play both the drums and flute during my senior year in high school. Now, that I am an older adult, working a lucrative full-time job, I was able to purchase these instruments and play them for my own personal enjoyment--and with no one from the BORG to tell we what to do!]
So, Akid48, give the above some thought. I would hate to see and hear that you have given up your dreams and desires because your parents are in actuality listening to a group of men who don't want anyone to enjoy LIFE!