The problem is here
AND studStatus=1));
There should only be one closing bracket
AND studStatus=1);
Whitespace can be your friend ... I pasted your query into SQLFiddle and inserted some linefeeds ...
SELECT u.Id as UserId,u.Nikname as Nikname,u.surName,
s.FormId,s.oldId,s.Mess,s.Date,s.Id as FrmId,s.pic
FROM user as u, `stud` as s
WHERE u.Id IN (
SELECT MId as Id FROM `Common`
WHERE studId='1' AND studStatus=1
UNION
SELECT studId as Id FROM `stud` WHERE Id='1' AND studStatus=1)
);
The error message then said the error was on Line 9 ... straight to the problem!