OK, I have experience of this issue. Having invested some time getting a Cordia rep to come and meet our Head Teacher and a small selection of parents, this is what I learnt.
Cordia are VERY clued up about the minimum levels of sugar, salt and saturated fat that they provide in their meals. They are legally obliged to adhere to these standards. Even their grimmest sounding burgers, hot dogs and pies have to adhere to these levels. They have to factor in these levels over the entire menu for the day so that main meal, accompaniments, "afters" and drinks in total do not exceed the limits. I was impressed by this and I have to admit, surprised.
Currently, EU laws do not legislate against the use of artificial sweeteners in our school meals (I'm actually unhappier about this than the use of "real" sugar).
The "Flavoured milk debate" is a hot one. Their take on it is that some children don't get calcium in any way shape and form and if they can get some milk into them on a daily basis, albeit flavoured, it's worth doing.
Now, I happen to live in an area where there is a high proportion of parents who care about what their children eat. They try to give them a healthy diet and are informed about health issues and diet. You will find that in areas like this there are a lot of kids going packed lunches. In many areas of Glasgow the situation is different.
The bottom line however is that if you can get enough parents in YOUR school to say categorically that they DON"T WANT FLAVOURED MILK then you can make this happen. I was told this by Cordia's representative. I was encouraged by this and then, as a school we tried to canvass opinion from the parents and this is where we fell down. The parents who felt most passionately that the provision of flavoured milk was "a bad thing" were sending their kids packed lunches anyway. Rather surprisingly, when our Head Teacher sent out a letter in every child's school bag asking for feedback (on the school meals as a whole rather than just the flavoured milk issue admittedly) the response was pathetic. My feeling is that many parents who sent their kids for school meals didn't think that giving them flavoured milk was a particularly bad thing 
So it's down to you. I would say, get onto your Parent Council, work with your Head Teacher to canvas opinion from the parents (and prepared to be surprised by the response possibly) and you could make flavoured milk disappear from your school.
FYI my child has started P3 and is now having a packed lunch with water and is NOT a happy bunny - he LOVED having chocolate milk every day 