My son is also in half-day kindergarten, and he is the only student out of 90 that opted for half-day! So he is also mixed in with full-day, out of necessity. The day is structured so that all of the necessary curriculum (basically reading, writing, arithmetic) is in the first part of the day. As a half-day student, he cannot attend any specials (music, art, PE, etc.) as those are scheduled in the second half of the day. I completely understand that mainstream thought prioritizes the three R's above "specials", but have a hard time understanding WHY. Of course we want our children to learn to read and add, but so much research clearly shows that we lag behind in education as a nation precisely because we start the fundamentals too early. Yet, we know that the prime age for music learning, like language acquisition, is from three to five years old. So I feel like music especially should be a higher priority in Kindergarten than math. I understand how frustrating it can be! And for us, full-day is a non-option right now because my child is still having a difficult time with the adjustment to Kindergarten, and we are two months into the school year already.
It appears your child's teacher has the option to structure the day differently for your child, and there is no reason that your request can't be considered, at the very least. You can always let her know up front that you understand if there is a scheduling conflict with the library or another school facility, but that otherwise you would appreciate it if she would consider changing her schedule so your child can attend specials. If you are willing to teach anything missed at home, what is prohibiting her from fulfilling your request? If you continue to meet resistance, it may be best to gently bring up the issue again via email or over the phone. I would definitely speak to her in the event that she makes an agreement to do things one way, and then doesn't even let you know why she opted to do differently.
BTW, it isn't as if you petitioned the school board and begged for a concession to be made so your child could attend half-day. The school gives the option, and just because very few people take the option doesn't mean that your child should be excluded from class parties. If the teacher is adamant about end of the day parties, would it be possible for you to bring your child back to the school on those days if you don't want her to miss them? Or would she consider some other arrangement to make it possible to include the half-day students?