Lunch-time is a wonderful opportunity to engage and learn about peer-relationships. While we all want our children to be fully integrated/immersed in the school-experience, lunchtime can prove tricky in that it is only loosely structured which may be problematic for autistic children.
Social stories about what happens at lunch, and about what is expected during this "period" (both general and specific expectations), can be beneficial.
If it is your concern that your child's eating habits are "different" from the other children, as was my concern years ago, you might want to try sending some "finger-type" foods like carrot sticks, or pickles, or cheese and crackers, or raisins, and of course, pieces of fruit — these are just a few suggestions.
One thing that our school did, that was of tremendous benefit to our son, was provide a lunchtime "file" folder with all of his classmates pictures (velcro-backed) in the folder next to their respective names. It was for a lunchtime "buddy-schedule" — each day, two of the children ate lunch with my son in an unoccupied classroom — it was not so noisy and distracting as the lunch area used by the majority. Further, it was a very effective strategy for promoting the interpersonal skills of all of the children …perhaps it is something that might appeal to you as a parent. If it does, hopefully it is an option that will be embraced by your school should you ask them to initiate such. Good luck to you and your family!