Try using these script steps, for example:
Insert Text [Students::DateOfRegisteration ; >= ]
Insert Calculated Result [Students::DateOfRegistration ; "01/27/2011" ] (make sure you uncheck 'Select contents')
FM is fussy about entering text (like '>=') in a date field by the Set Field script step.
If it's literal text not a date in a field or variable, I'd just take out the ampersands:
Set Field [Students::DateOfRegisteration ; ">= 01/27/2011"]