It sounds like your screen isn't refreshing to update immediately after you select a student from the drop down. When you created the portal, did you add the Full Name field from your student table or is it a calculation field in Ensemble Participant that returns the full name from the Student table? I believe that you've used the first approach.
Check your Table Occurrence names in your relationship graph (the name at the top of each box). Make sure that the correct TO name was used when you specified the Full Name field in the portal. (It should match the TO title used when you set up the relationship linking the Student data to the Join Table.)
I'm not sure, but You might get a difference in behavior if you find you refered to a different TO that also refers to the Student table.