documentclass{article}

usepackage{booktabs,dcolumn}
usepackage{siunitx}

newcommand*mc[1]{multicolumn{2}{c}{Eleverne fra $9$.~#1}}

begin{document}

begin{table}
defspc{hspace{0.8em}}
centering
 caption{Something.}
 label{tbl:1}
  begin{tabular}{
           S[table-format = 2.1]
    >{spc}S[table-format = 2]
           S[table-format = 3]
    >{spc}S[table-format = 2]
           S[table-format = 3]
  }
   toprule
    {S{o}vnm{ae}ngde} & mc{A} & mc{B}                 midrule
    si{hour} & {Abs.} & {Rel.} & {Abs.} & {Rel.}        midrule
     6.5       &  1spc &   4    &  0spc &   0             7         &  4spc &  16    &  2spc &  10             7.5       &  3spc &  12    &  3spc &  15             8         &  8spc &  32    &  9spc &  45             8.5       &  5spc &  20    &  3spc &  15             9         &  2spc &   8    &  3spc &  15             9.5       &  1spc &   4    &  0spc &   0            10         &  1spc &   4    &  0spc &   0           midrule
               & 25spc & 100    & 20spc & 100           bottomrule
  end{tabular}
end{table}

end{document}

Overfull hbox (8.00003pt too wide) detected at line

documentclass{article}
usepackage[utf8]{inputenc}

usepackage{booktabs,dcolumn}
usepackage{siunitx}

newcommand*mc[1]{multicolumn{2}{c}{Eleverne fra $9$.~#1}}

begin{document}

begin{table}
centering
caption{Something.}
label{tbl:1}
begin{tabular}{
  S[table-format = 2.1]
  S[table-format = 2]
  S[table-format = 3]
  S[table-format = 2]
  S[table-format = 3]
}
toprule
{Søvnmængde} & mc{A} & mc{B}          \
midrule
si{hour} & {quad Abs.quad} & {quad Rel.quad} & {quad Abs.quad} & {quad Rel.quad} \
midrule
 6.5 &  1 &   4 &  0 &   0 \
 7   &  4 &  16 &  2 &  10 \
 7.5 &  3 &  12 &  3 &  15 \
 8   &  8 &  32 &  9 &  45 \
 8.5 &  5 &  20 &  3 &  15 \
 9   &  2 &   8 &  3 &  15 \
 9.5 &  1 &   4 &  0 &   0 \
10   &  1 &   4 &  0 &   0 \
midrule
     & 25 & 100 & 20 & 100     bottomrule
end{tabular}
end{table}

end{document}