I work in a secondary school, and our rule is - in classrooms, water in clear plastic bottle only. We permit flavoured water, but other than that, it gets confiscated. It'll be in the school rules somewhere, which you will have agreed to as part of your home-school agreement.
Reasoning for water only is easy - hard to mask anything else in there (even flavoured water), whereas squash or similar could quite easily contain lots of sugar or similar that may affect a child's attention span.
Not saying its morally right, just that there's little you can do about it (unless enough parents kick up a fuss) other than get flavoured water.
Genuine question... I have a legal duty to educate my children and so does the local authority. But what happens if I disagreed with the school rules and refused to sign them? They obviously wouldn't be allowed to join the school. But considering a legal duty on both sides then what happens?