How may school stress or other stress affect a child's performance at school?

A lot. Kids seem to be over "busy" with activities, school work and sports that fill all their days and waking hours. Keep in mind children need time to "unwind" just like adults and that sometimes they need time to do, well nothing or just have unstructured play time. Talk to your child to see what is bothering her. Revisit the activities and see what you need to do to decrease stress and improve school.