Two issues here.
The first is informational. If the packed lunches are somehow nutritionally lacking, instead of just attacking them as inadequate, how about, as you suggest, providing printed guidelines to parents so that they can improve their content, control portion size, etc? Then get food manufacturers aboard with producing conveniently packaged, nutritional food items that meet the guidelines. It is not like they (the food manufacturers) won’t respond and quickly. Supermarkets are full of products designed for lunch boxes/bags (school and office) right now.
The second issue is more sinister and has to do with attacking any input from outside the school as uninformed, inadequate, and harmful. This is the left/liberal education agenda in action to isolate and indoctrinate students that only those things provided through approved government channels are useful. Packed lunches, no matter how nutritious, will, prima fascia, always be inadequate because they represent individual resistance to the educational system. They create difference when the objective is uniformity. Better all children be required to eat the same barely palatable school lunch than social dissonance be created when students see what their individual lunches contain and some dare to complain to their parents about it.