First, I am impressed with all the comments under this question and the wide range of experiences.
Next, it would be interesting to see a copy of this district nutrition policy and how it describes what food may be brought from home. As a Licensed School Nurse in the state of Minnesota, I assisted with development of policies regarding food allergies. I cannot even imagine what the developers of your son’s school policies must have been thinking or intending when they wrote in that no child may bring nutritional supplement drinks for lunch. I would be interested in their rationale.
Moreover, how do they police “bag lunch” contents that a student brings from home? Does the teacher inspect each bag? If the liquid supplement were the only thing coming in the lunch each day I might ask the parent about it. In most school settings, there is not time or resources sufficient to police the food a student brings to school. For example, schools can declare “allergen free” or “Peanut free” zones or classrooms, but you cannot guarantee there will not be the allergen present in the environment. Schools are public places, many are open to people in the evening too. In addition, kids are kids and will bring in things parent do not know about. Therefore, for school personnel to supervise cold lunches brought from home, they must have a tremendous amount of resources available.
A few years ago, the USDA National School Lunch Program changed some standards. It was quite clear on what could be on a school lunch menu and what must be placed on the student’s plate. There were items a student could not decline unless there was a written medical authorization to change the items due to a diagnosed medical problem.
You can read all about the program at the link above.
To my knowledge, the program guidelines outline the school lunch and it does not dictate what a student can bring to eat at lunch from home. I agree with the writers who declare that what is in a student’s cold lunch is the parents business and prerogative unless it is grossly imbalanced or inappropriate on a regular basis. The nutritional supplement may be included in an IEP or a 504 plan if medically necessary, but generally, you only want things on an educational plan that state if the student did not have that service or accommodation they would not be able to take advantage of a free public education in a least restrictive setting.
What happens if your child decides not to eat his bag lunch? Does he end up going to the school health office and calling you? Does he get a headache? Does he need to go home on a regular basis? How much, and how often, does it impair his ability to take advantage of school if he does not have the nutritional supplement?
If the school principal will not hear what you have to say, contact the superintendent of schools. If you do not feel heard, then contact a member of the school board or go to an open board meeting and ask your questions. You are probably not the only parent with questions on the policy.