When posting code, please put the code between [code][/code] tags as it aids readability.
I suggest that you pay more attention to quoting. e.g.
sed -n -e /CS261/CMPTR261/p' $i > corr_$f;
sed -n -e '/CS261/CMPTR261/p' $i > corr_$f;
Also you have used the '>' redirection. This will overwrite any existing file with the same name.
Use the '>>' redirection instead, so that if the file does not exist, it is created. Or, if it does exist, then the output is appended.