Table 1 Description of the participating and non-participating schools [n, mean ± standard deviation (SD) for continuous variables or n (%) for categorical variables]

Responsible authority

0.0002

 Public (Municipality)

62 (61.4)

342 (78.8)

 Private

39 (38.6)

92 (21.2)

County

0.5347

 Stockholm

51 (50.5)

234 (53.9)

 Other

50 (49.5)

200 (46.1)

Average number of students per teacher (SD)

13.3 (2.7)

12.9 (2.4)

0.1914

Average proportion of teachers having University degree (SD)

79.1 (11.9)

81.6 (11.2)

0.0416

Proportion of teachers with University degree

0.3767

 High (>87)

28 (28.0)

145 (34.7)

 Medium (78.5–87)

34 (34.0)

139 (33.3)

 Low (<78.5)

38 (38.0)

134 (32.1)

Average proficiency score of students in the 9th grade in 2014 (SD)

221.8 (25.4)

217.3 (26.3)

0.1261

Mean number of students in school (SD)

371.8 (185.5)

399.0 (196.6)

0.2106

Number of students in school

0.7643

 High (>432)

31 (31.3)

145 (33.7)

 Medium (279–432)

32 (32.3)

145 (33.7)

 Low (<279)

36 (36.4)

140 (32.6)

Average number of students per grade (SD)

64.2 (34.7)

69.9 (38.4)

0.1708

Average number of students per grade

 High (>78.4)

32 (32.3)

145 (33.7)

0.8875

 Medium (47.1 – 78.4)

32 (32.3)

144 (33.5)

 Low (>47.1)

35 (35.4)

141 (32.8)

Average proportion of students with foreign background (SD)

19.4 (16.5)

25.5 (22.0)

0.0031

Proportion of students with foreign background, tertiles

0.0383

 High (>24)

22 (23.9)

143 (35.0)

 Medium (12–24)

31 (33.7)

144 (35.2)

 Low (<12)

39 (42.4)

122 (29.8)

Average proportion of students with parents with university education, (SD)

55.2 (16.5)

52.6 (17.2)

0.1879

Proportion of students with parents with university education, tertiles

0.3580

 High (>61)

33 (34.0)

137 (32.3)

 Medium (44–61)

37 (38.1)

138 (32.6)

 Low (<44)

27 (27.9)

149 (35.1)