Well, the idea is that the calender week of the current date minus the
calender week of the semester's start date plus 1 would give you the
relative week in the semester i.e.

Something along these lines:

sem start cal week sem week
27.02.2006 10 1

current dates
06.03.2006 11 2 <-- =WEEKNUM(A5;2)-WEEKNUM($A$2;2)+1
13.03.2006 12 3
20.03.2006 13 4
27.03.2006 14 5
03.04.2006 15 6

Hans