Flavored milk (such as chocolate milk) is a favorite with many kids who eat school lunch. What's not to love about the sweet taste of icy cold chocolate milk?! It IS after all, a good source of calcium and vitamin D and it packs 8 grams of protein per cup.
That said... chocolate milk is sugary and is best considered as a "treat". The chocolate milk "treat" might not seem like much in the scheme of things but think about all of the other "treats" that might creep in throughout the day... like that box of sugary cereal with school breakfast and maybe the pack of gummy fruits eaten as an afternoon snack. And let's not forget the sugary granola bar that might be eaten a couple of hours after dinner. Some kids end up eating multiple servings of treat items each day!
On a side note, chocolate milk often contains "carrageenan". This controversial added substance prevents the chocolate from separating out of the milk and ending up in the bottom of the carton (whatever happened to shaking it up before we drink it?!).
So... while treats are fine to have on some days, consider drinking water or unflavored (plain) milk on most days with school lunch.