Over the past few years soft drink companies developed milk, juice, water, sport drink based products specifically targeted for schools. The problem however is two fold. The kids don't drink them (unless you load up with sugar, thus negating any health benefits). Second, as the schools have their budgets cut, they lean on sales of soft drinks to stem the bleeding. They (schools) set their own prices for the soft drinks and make a ton of money off them if they sell, and soft drinks sell very well. I'm not going to blow smoke and tell you that this doesn't benefit Coke/Pepsi/Cadbury. They don't make a lot of money off of the education deals and in fact lose money in some cases, but you do get brand identification.
As I stated all of the soft drink companies are feverishly trying to develop healthy alternatives to their flagship offerings, but the reality is people just don't drink them. As for banning it in schools...I agree with you at an elementary/middle school level or stick with the water/juice vendors. High school kids tend to get it at other locations if the vendors are shut off, so it really wouldn't curb their intake.