It is probably fortunate if the poor kid gets to attend a better high school than most high schools that poor kids have to attend. College admissions readers will be more confident about his/her high school preparation (assuming that grades and course rigor are not too low), but may still take into account other stuff related to being from a poor family (e.g. EC is working to help support the family or caring for other family members instead of typical expensive ECs that are markers for high SES).