It sounds correct to me. It is explaining that you cannot eat in the classroom, but have to do your eating in the dining room. Dining should only have one "n" in the middle, though.

I also would use 'You can only eat in the dinning hall' rather than 'You must eat in the dinning hall' but using 'must' isn't necessarily grammatically incorrect. I think the prior sounds more appropriate because depending on the tone 'You must eat in the dinning hall' doesn't really state that the dining hall is the only place you can eat in.