If the edit box contains just a single value for one day of the week try using =="T" to find just those records with Tuesday specified.
Well my problem is that it contains the letters for every day of the week the student attends, for example "M T F" or "W Th F."
Then search for "T ". Include the quotes and a space after the T.
And for data entry and display purposes, this could still be a check box formatted field, but if you do, the find criteria would be: ="T"--something that you could specify in a scripted find or by using a copy of the field formatted as an Edit box if you need to do a manual find.