>1 colleges names r in dropdownlist controll
>2 name of student in another dropdownlist

That doesn't explain how you know to which college a student belongs.
IAC, I assume from this that you have a table containing the names of the colleges so why not add a field for current user count.
When a student identifies themselves check the count. If it's not at the limit then increment and allow access.
Decrement the relevant count in Session_End.......