Is it ethical for schools to offer merit scholarships, when these are more difficult for students from some socioeconomic backgrounds to get?
Schools don't charge more to low performing students, they provide opportunities to attract and retain high performing students, through merit-based scholarships and grants. While it is probably true that low-SES individuals are less likely to qualify for such awards, it is not fair to surmise that schools are 'charging more' to low-SES individuals.  There are many 'rich' students paying the same tuition as their less-well-off peers.  

The more pertinent question is whether there exist sufficient non-merit based scholarships for disadvantaged populations, and need-based financial aid, to give low-SES individuals opportunity (probably not).