Ds1 is a horrendously fussy eater. When he started reception, like you I hoped giving him school dinners (which look really yummy, they've definitely been JO'd! ) would extend his repertoire. So I let him have packed lunch Tuesday and Thursday, and SD Mon, Wed, Fri. I also thought this would let him mix with different groups of children.
What actually happened? Well, on the days he had school dinner he ate nothing. For half a term. The reception teacher took to sitting with him, he would make a choice of food, and sit there, sadly explaining to his teacher that he just couldn't eat anything on his plate. No improvement over half a term, after which I had a chat with his teacher, as she was really anxious about it (he is very skinny at the best of times, so doesn't have much in reserve!). The only exception was Friday, when they had fish and chips, which he wolfed down, so now he has packed lunch Mon-Thurs and SD on Friday. His best friend does the same pattern, which is nice for them.
He is still ultra fussy (now in Yr1), and on Fri he very rarely feels able to eat the pudding, but he does eat the fish which is great. I do give him a healthy(ish) packed lunch, and he eats moderately healthily (if a limited range) at home, but it just wasn't going to work at school.