I’m coming into this discussion rather late, but I believe that the main reason for cutting down on the cola (and other sodas), is that a) they are empty calories and b) the enormous amount of sugar in even a single can may have an adverse effeect on the students, especially if they are drinking soda after soda. The caffeine may also be a factor. It is better for the kids to be drinking juice, milk and just plain water. Cola has a dehydrating effect, as the caffeine acts as a diuretic. I happen to love soda (only drink diet, though), but even I have cut down considerably to only drinking an occasional soda on Shabbos, or at a simcha.
As to the rules and regs in Yeshivahs, well, it IS true that if you can’t stand the heat get out of the kitchen. You don’t like this Yeshivah’s rules, send your kids elsewhere. But what if there IS no “elsewhere” readily and easily available to a parent?
What if they have sent their kids to that Yeshivah for years, and suddenly the Yeshivah changes many of the rules that were never in effect before? I experienced that to a very mild degree when my daughter’s yeshivah suddenly decided they wanted to institute a “uniforms” policy. After having bought clothing for the school year, I had to now suddenly spend way more money buying the uniform. I had no choice but to do so. There was no other school to which I would be able to send my daughters, for various reasons, both hashkafically and in terms of the friends they would have in their school versus another. Sometimes, you have to bite the bullet in order to benefit in other ways.