Find out who runs your cafeteria and ask them to meet with you about the issue. They will probably be happy to do so. Here are some things you might want to mention in the meeting:
Schools around the country are responding to student demand and health recommendations by incorporating more veg options. Entire public KE2 school districts in Los Angeles, Miami, Detroit, Buffalo, and other cities have implemented Meatless Mondays.
Vegan options are inclusive; they suit people with many different dietary restrictions: people with varied religious and ethical beliefs, allergies, lactose intolerance, or a desire to slim down.
Adding veg options is a good way to promote healthy eating among students, and is also more environmentally friendly and sustainable.
If you are required to purchase a meal plan at school, you are essentially being forced to buy food that you cannot eat.
Keep in mind that changing your school cafeteria may take time and effort. Your dining director is probably extremely busy, so offer to help out. That could mean suggesting meal ideas, promoting the changes to other students, or anything else that might be needed.