I have a flexible schedule, which allows me some flexibility if there's a "show up for half an hour first thing in the morning, or for 30 minutes at the end of the day" thing. Otherwise, for things like field trips I'll take a vacation day.
When you say you want to be involved, what do you mean? I was so upset when I missed one of my daughter's field trips when she was 4. Up until that point I'd been to Everything. You know what? She didn't care. She was fine with me going to some things and not to others, because very few parents made it to everything. So if I make to half of her field trips this year, I know she'll be fine.
If you want to help with the teacher & the classroom but just can't manage during school hours, you can always offer to help with things outside of school hours. Maybe the teacher needs someone to help organize the books, set up for a messy craft the next day, cut out a million tiny little circles ... you could offer to be one of the people who helps with that sort of thing.