I think it would depend on your district policies. In my district the cafeteria does NOT know about any allergies until the parent turns in a special form signed by a physician. Parents are notified of this during registration every year if they select a student has allergies/life threatening food allergies - parents are directed to a link that provides all the necessary forms and instructions. Once a parent/physician completes the form - it is on file for the remainder of the students education through 12th grade while enrolled in the district.
I will also remind parents as they hand in emergency meds/Epipens to me yearly, that the cafe does not know about the allergies unless they receive the form signed by the doctor.
At the beginning of the school year, my cafe manager gives me a list of those that have turned in the form. I then go and check my list of allergy kiddos and send a reminder to the parents about getting the form turned in so the cafe knows what not to serve to their kid. After that its on the parents to make sure the cafe has the information.