How can I get more people to eat in my school diner?

Start with some marketing research. Do a survey or a focus group (or two or three) and ask things like how often students eat there, why they don't eay there more often, what they like and don't like about (food, service, theme/atmosphere, seating, prices, etc.) Give students a chance to comment, not just choose from multiple choice answers and rankings.

If your school has a marketing class, maybe you can get some students to do this for you for free (or for meals) as a class project. Talk to the marketing professor about it.

Then devise a plan to improve the areas where your place falls short. If you use marketing students, they can help with this, too.

Publicize the changes and the diner in the student newspaper, on bulletin boards and kiosks, etc. If there's a student store on site, ask to insert diner flyers into bags for a week. The flyers should have the diner's name and logo, hours and location. If need be, add a little map. Put a coupon on the flyer, too, maybe offering a freebie rather than 10% off. For example, "Free beverage with any sandwich or entree." or "Free dessert with any meal" Or "Free side with any sandwich."

IMO, the most important factors to students are (in about this order):
price
decent food quality
cleanliness (no bugs, no strong disinfectant smells)
range of foods available
reasonably fast service, reasonably friendly

Theme is probably not the reason for lack of business. It's usually price, sanitation/cleanliness, anf the food itself.

The fresher and higher-quality the food, the better. This includes the oil. Don't reuse the same oil a thousand times - it makes the food taste awful.

The standard sandwiches, salads, soups (in season) are always in demand. More people are vegetarians, so offer some really appealing veg choices. Burgers and melts are also still loved, as are entrees. Also pizza, neverendingly.

Sides you could have: French fries and onion rings, but also potato salad, macaroni salad, cole slaw, garden salad, carrot pineapple salad, broccoli slaw, cooked greens, cooked carrots, cooked green beans, etc. Fresh fruit salads in season.

People are health conscious, so offer some lean options - but not the old diet platter of cottage cheese and canned fruit on lettuce. For example, you can stir-fry strips of lean beef or chicken, or shrimp or tofu, with vegetables and serve over rice or pasta in a sauce. This can be completely customized to each person - the pick which veggies, protein, and sauce they want, and rice or pasta. Sauces can be teriyaki, brown sauce, Hunan, spaghetti, alfredo or pesto. (Some employee cafeterias have this nowadays, and it's very popular.)

Experiment by trying things out as daily or weekly specials. You can even do this around themes.

One more suggestion. If you don't offer this already, make it easy to get carry out.