sub select_your_school {
    my $namesref = shift;

    # create a list of sorted hash keys
    my @menuitems = sort keys %$namesref;

    print $q->header,
          $q->start_html(-title=> 'Select Your School'),
          "Please select your school from the following list:",
          $q->start_form,
          $q->popup_menu( {-name =>    'school_id',
                           -values =>  \@menuitems ,
                          }),
          ....
[download]

Or you could do it in one step:

sub select_your_school {
    my $namesref = shift;

    print $q->header,
          $q->start_html(-title=> 'Select Your School'),
          "Please select your school from the following list:",
          $q->start_form,
          $q->popup_menu( {
              -name =>    'school_id',
              -values =>  [ sort keys %$namesref ],
                          }),
          .....
[download]

Various sorting options:

# Sort by alpha
sort keys %$namesref

# Sort by alpha reverse order
sort { $b cmp $a } keys %$namesref

# Sort by numeric value
sort { $a <=> $b } keys %$namesref

# etc...
[download]