In the United States, we would say "he was left back a grade" or "he repeated a grade." Also, in the United States, educational theory has varied from time to time and from place to place. Sometimes, the custom has been "social promotion," in which all students move from grade to grade together, regardless of performance, with the weaker performers getting "remedial work" and the stronger ones getting advanced "enrichment" materials. The idea was that the harm to a student's social development of not staying in his own age group outweighs the harm of taking lessons that are beyond his ability. At other times and places, underperforming students are "left back a grade" or "repeat a grade," while gifted students are allowed to "skip a grade."

Véase la siguiente página para ver como algunos traductores han vertido "retraso escolar" al inglés. https://www.linguee.com/spanish-english/translation/retraso+escolar.html