The primary cause is a lack of qualified teachers.
Students with Somali, Roma, or Arabic as their first language are most affected.
Three of four municipalities have also had difficulty recruiting instructors to lead mother-tongue classes.
“We’re not going to let ourselves accept such ‘excuses’ from the municipalities. They must show that they are trying to recruit,” said Alf Johansson, a lawyer with the inspection division of the National Agency for Education.
250 of Sweden’s 290 municipalities participated in the survey.