In Regents v. Bakke, the Supreme Court held that racial quotas were impermissible but that schools were permitted to have race be one of many factors that were considered when colleges considered an applicant.
So, a 10 percent threshold would likely be found to be unconstitutional.
In Athens, a person or group of people could be ostracized for up to ten years, meaning they were banished from the city. Usually this was meant to protect the city and its inhabitants from harm, or if the person was accused of being a tyrant (a cruel and oppressive ruler)