You can do it with one query (and with better security) by replacing your foreach loop on the id param with something like:

$sql = "DELETE FROM student WHERE student_id IN (".
       join(",",map {$dbh->quote($_)} $cgi->param('id')).
       ")";
$sth = $dbh->prepare($sql);
$sth->execute() || die $DBI::errstr;
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You really should be running CGI scripts in taint mode (-T); it will help you catch common but serious errors like forgetting to quote input from forms.