Education levels

The median income for high school dropouts in the U.S. is 19 thousand a year. The median for graduates is 29 thousand. Why isn't there a provision requiring high school level equivilancy in English before granting a visa or green card. In 1900 there were just 10 million Mexicans on the planet. Most immigrants from Mexico are uneducated in Spanish and are not of the caliber that will secure the future of this country.