How well can I plan?

So this afternoon I knew I was going to have a challenge with my year 8's. I sat down and planned the entire class period in 15 to 20 minute blocks to fill 80 minutes. I really couldn't have had a better map for the class period. I knew exactly what I wanted to do/say and what I wanted the kids to do and how far they should get with their work. Pretty much, it looked like a fool proof plan...however, looks can be deceiving and I know that very well. Not a single part of my plan went as planned. Students kept interupting, asking the same questions, not following instructions, getting smart, mouthing off, in other words: making my life a complete frustration! Not a single person went along with what I was aksing. How am I supposed to teach music if I have to do crowd control the whole time? This is just silly, rediculous actually.