NEW YORK (AP) _ Thirty-seven percent of more than 1 million high school football players in the United States were sidelined at least once during the past season by injury, the National Athletic Trainers Association said Thursday.

In a news conference in New York, the association said that its study showed there were more than 500,000 high school football injuries that resulted in a one-time loss of playing time during the 1987 season.

Sprains and strains accounted for 49 percent of the injuries, the study showed.

Of an estimated 1,021,000 youngsters who played high school football last year, 374,678 sustained at least one injury that cost them playing time. The survey showed there was a 5 percent increase over the previous year in major injuries, those that sidelined a player for three weeks or more.

Projectill, who directed the study at San Diego State University.

''We projected 21.7 percent fewer minor injuries, which accounted for most of the decrease,'' Powell said. ''Results from the second year of similar studies we did in college and professional football also showed a decrease. We have reason to believe it is more a reflection of record-keeping than a reduction of injuries.

''The actual injury toll is probably somewhere between what we projected last year and this year.''

Of the injuries, 25 were classified as minor injuries, six were moderate and four were major.

Based on the study, the average high school football program can expect an average of 2.4 injuries per week during the season. Most accidents occur during practice.