Let's start with the one thing we seem to agree on:
Why is the only responsible thing to mandate everyone does what you want?
We shouldn't mandate that everyone does what I want. Or what you want. We should mandate what is necessary for a lot of people to share the same space.
In a survey of Floridians (not specifically parents of school age kids), 2/3 thought there should be a mask mandate in schools. For yesterday's first day of school in Orange County, 95% seemed to support masking. When the majority wants to follow the CDC advice for creating a safe classroom, those few who insist on going maskless are not simply making themselves more comfortable - they're also ruining it for everyone else.
Are they really effective for long term indoor exposure? I think it was pretty well established that the benefits of masks are greatly reduced indoors the longer the exposure.
You're right - there are conditions that can raise or lower the effectiveness of masks. But we're not able to control all those conditions, like holding classes outdoors instead of indoors.
Vaccines would be the optimal tool for stopping transmission. Until we have vaccines for young children, we have to ask what are our next-best options? These include good ventilation, social distancing, handwashing, and masks. These are additive, that is, each one alone is not that great, but when we rely on all of them together, they add up to pretty good protection.
If a group of people feel that's in their best desire, they're more than welcome to wear masks. Why is it everyone else's responsibility?
Because we all share the same air supply. If everyone is masked, the viral content of the air is lower. Each person who unmasks may potentially raise the number of viral particles in the air we all breathe. If someone wanted to come with their own oxygen supply, they wouldn't have to mask. Other than as needed to use their own oxygen, of course.
Just like people who prefer to drive 30 MPH will be asked to get off the highway because by following their own personal desire they make the highway unusable for everyone else.
Maybe we should ban masks because some people feel their kids are negatively affected by not being able to see each others faces and communicate properly?
No the justification for masks is based on the danger from the virus, and the need to communicate properly doesn't obviate that danger. If the use of masks creates other problems, such as communication, then we do need to address those, but not by lowering our protection from the virus.
Consider someone with a serious disease, for whom life-saving treatment may entail losing their hair. The loss of hair is a problem, and should be addressed (wig, hat), but not by forgoing the life-saving treatment.
Keep in mind that the mask mandates have been put in place for 2 weeks in some schools, a month in others. The idea is to use them whenever the case rate is high in the local area. They are not necessarily to be used for the entire school year, and in many classrooms there are frequent breaks and increased outdoor activities so that masks aren't used 5 hours at a stretch.