Gantt chart help

Dear Friends,

I am a school teacher and preparing a utility for my students to create a Gantt Chart.

We got a problem that we follow a ten days time table (Wk1, Wk2). Groups got different timetable arrangements e.g. Group A got 2 lessons in wk1 and 3 in wk 2, while Group B got 3 in wk 1 and 2 in wk 2.

Two show this on the Gantt chart. I need to merge two cells/three cells as required depending on the wk1/wk2 e.g

IF WeekNo = 1
Merge Two cells
If Task not Completed
bgcolor = Yellow
else
bgcolor = Green


IF weekNo = 2
bgcolor = Yellow
Merge Three Cells
If Task not Completed
bgcolor = Yellow
else
bgcolor = Green

I will be obliged.....