n this case, "at school" and "in school" have slightly different meanings:

"At school" typically means being physically present at the location (the school building) but doesn’t necessarily mean you were there to study. So, "I ate rice, drumstick, and some greens at school" is correct if you're simply talking about where you ate.
"In school" often refers to being enrolled or actively involved in school as a student. For example, "I am in school" implies that you are a student.
Since you’re talking about where you ate, "at school" is the correct choice.
【 在 jackadandy 的大作中提到: 】
: 南非人，母语英语，口音也很好，在苏州一个小学教了几年英语了。特别活跃，肢体表情夸张，用生命上课的感觉，感觉挺好的。直到她问娃学校的午餐怎么样，娃说，today i ate rice, drumstick and some greens at school.然后老师一直纠正她说用in school, i ate....然后后续娃表述在学校做什么活动时，老师再次纠正用in school.听的我和娃有点晕，直觉老师说的不对，仔细查阅了一下，这种情况下确实应该用at school.但感觉这种错误是不是太显而易见了，母语老师不应该犯这个错误啊
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