Sort a dataset of school IDs and group IDs into pairs of the same group, but where school ID must be different

pairfind.php

<?php

$pairs = [];
$dataSet = [];

const SCHOOL_ID_COL = 0;
const GROUP_ID_COL = 1;

// load data
$fileName = __DIR__ . '/matching_problem.csv';
$fileHandle = fopen($fileName, 'r');
while (($row = fgetcsv($fileHandle)) !== FALSE) {
    $dataSet[] = [$row[0], $row[1]];
}
fclose($fileHandle);
if (!is_numeric($dataSet[0][SCHOOL_ID_COL])) {
    unset($dataSet[0]);
}

// sort by order of school ID frequency
$schoolFreq = array_count_values(array_column($dataSet, SCHOOL_ID_COL));
arsort($schoolFreq);
$schoolFreq = array_keys($schoolFreq);
$sortedDataSet = [];
foreach ($schoolFreq as $schoolId) {
    foreach ($dataSet as $student) {
        if ($student[SCHOOL_ID_COL] == $schoolId) {
            $sortedDataSet[] = $student;
        }
    }
}
$dataSet = $sortedDataSet;

// loop through all groups and find pairs
$groupIds = array_unique(array_column($dataSet, GROUP_ID_COL));
foreach ($groupIds as $groupId) {
    while (!is_null($firstKey = getNextStudent($dataSet, $groupId))
        && ($schoolId = $dataSet[$firstKey][SCHOOL_ID_COL])
        && !is_null($secondKey = getNextStudent($dataSet, $groupId, $schoolId))
    ) {
        $pairs[] = [$dataSet[$firstKey], $dataSet[$secondKey]];
        unset($dataSet[$firstKey], $dataSet[$secondKey]);
    }
}

printf('made %d pairs', count($pairs));

function getNextStudent($dataSet, $groupId, $excludeSchool = NULL) {
    foreach ($dataSet as $key => $student) {
        $groupMatch = $student[GROUP_ID_COL] == $groupId;
        $schoolId = $student[SCHOOL_ID_COL];
        $isAllowed = !$excludeSchool || $schoolId != $excludeSchool;
        if ($groupMatch && $isAllowed) {
            return $key;
        }
    }

    return NULL;
}