I couldn't say... try simplifying your query and running it directly in phpMyAdmin to debug.
does this query return the results you expect?:
SELECT MIN(perfgroups1to4.date) AS fromDate , MAX(perfgroups1to4.date) AS toDate
FROM perfgroups1to4 INNER JOIN oxadmain ON perfgroups1to4.studid=oxadmain.stuid
GROUP BY studid
ORDER BY name ASC