For more than two decades, education authorities have warned that the reading, writing, mathematics, and science skills of America's young adults are insufficient to maintain its economic strength. Some indicators suggest that the average educational achievement of high school students in the U.S. is mediocre in comparison to that of other industrialized countries, and that it is not improving significantly. Moreover, there are wide differences in educational achievement among demographic groups. Authorities have recommended many different strategies for improving the educational achievement of high school students. One such strategy is to encourage them to take more rigorous college-preparatory courses and to earn higher grades in these courses. We studied the effectiveness of this strategy using data from students who took ACT's EXPLORE test in eighth grade, the PLAN test in tenth grade, and the ACT in eleventh/twelfth grade. The outcome variables in the study were students' ACT scores in English, Mathematics, Reading, and Science. The predictor variables were students' background characteristics, their previous educational achievement (as measured by their EXPLORE scores), the high school they attended, their course work, their course grades, and variables related to the context in which they took the ACT. We constructed models for predicting the outcome variables from the predictor variables. We then used the models to estimate the proportion of students who, under various scenarios of enhanced preparation, would have ACT score levels indicating that they were adequately prepared to take typical first-year college courses. The principal results are as follows: (1) Students' background characteristics, EXPLORE scores, high school attended, high school course work, and high school grades are all related to ACT scores, but EXPLORE scores are by far the most strongly related; (2) Improving EXPLORE scores is likely to be more effective in improving ACT scores than other forms of enhanced preparation; (3) Taking more standard or advanced courses in high school and earning higher grades is more beneficial to students who have high EXPLORE scores to begin with; and (4) There is significant variation in high schools' average ACT scores, even after accounting for differences in their students' characteristics. The benefit of additional standard course work, advanced/honors course work, and higher grades also varies significantly among high schools. The report concludes with a discussion of implications of the results and recommendations for additional research. Statistical Tables are appended. (Contains 11 tables, 2 figures and 19 footnotes.)