Do private schools not have the right to enforce their own set of rules and regulations, even if they conflict with what "modern" society deems acceptable? Her school makes no pretense about that fact that it is an Orthodox school bound by halacha - which even if you disagree, means the halacha of the 21st century which includes such things as kol isha. Would you similarly complain about an Israeli Arab Muslim school suspending a girl for not wearing a hijab? These are difficult issues to be sure but I think you are forgetting that the girl's parents voluntarily entered their child into a school that is governed by these rules...why feign shock when these rules are enforced?
Obama to meet with Jordan's King Abdullah in White House later this month (Reuters)
from the article: Judaism should inspire women's voices, not silence them