Hi Luis,
Unfortunattely I have limited time, but please have a look to this post:
Just copy the document and you can see how this user created a structure to organize an inventory.
Hopeful this will give you the right direction.
TIP: on the way I see your model: (try to draw it on a piece of paper, till you get all parts connected )
- Create one table with all the sub-tasks
- One or more subtasks are part of a main task
- Create one table with categories, like Impress, Writer, Calc. Each main task gets assigned at least one category
- Create one table with students and details
- Create one table where you asign to the students name more tasks/ sub-tasks, in this table you will assign the score achieved and maybe also the date.
Step by step you will see that you are able to build the app yourselve with some support of the community.
Success, 
//JP