Yeah, I agree with MusicianDad. I wouldn't put any reason why you won't be there. A simple "I am unable to attend" is much more polite. If they follow up or ask why, you can explain your reasons then.
I also wouldn't go.
I would be fine with your reasoning. I wouldn't want my first grader to miss school that early in the year. They're usually going over rules and routines that early on and I wouldn't want him to miss out and then later find out he was doing something different than what was explained the first week.
I found a piano teacher who teaches young children regularly and it's going really well. He's just had a couple lessons but I feel really comfortable with her teaching style and the way she explains things to him. He gets so excited and bounces around. He also loves practicing when I remind him.
We're going to start in a couple weeks.
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I'll also sometimes do stuff from the Learn at Home series.
He takes karate and piano lessons as well. :)