People get teased for lots of reasons. Sometimes it's for something as silly as the color of their hair or as ordinary as wearing glasses. It helps to understand the reasons why some kids tease others. For example, some kids tease others to get attention. Others may tease or bully their classmates because they feel bad about themselves.
Some kids think they are just being funny.
They don't really understand food allergies or why it's so important for a food-allergic person to stay away from certain foods. Ask him or her to stop. You can either talk with him or her alone or ask your teacher if you can tell the class about food allergies. Most likely, he or she will soon become bored and leave you alone. Act as if what he or she is saying doesn't bother you.