These statistics are cited by Samuel Huntington in Who Are We: The Challenges to America’s National Identity. They come from the 1990 census.
Percentage of all Americans except Mexican-Americans in 1989-90 without a high school diploma was 23.5%.
Percentages of Mexican Americans without a high school diploma in the same years:
First generation: 69.9%
Second generation: 51.5%
Third generation: 33.0%
Fourth generation: 41.9%
That is, in every generation, the Mexican-American high school dropout rates were higher than the national average, and in the fourth generation of Mexican-Americans the dropout rate was actually worse than in the third generation of Mexican-Americans.