Re Assign/ Reset Numer Field?

Hi there,

Regarding my dance classes database:

The dance classes are split up into categories, and within each category there are a number of dance classes.

I have an 'Order Number' Field. Now, each class has a number assigned to it,which is listed in the 'syllabus'.

Every year some classes are deleted etc, so some numbers will be deleted with the field.

My question: is there a way I can re-organise the number field, so the fields will run incrementely within the category(?).

E.g 1, 2, 3, 4, 5 and 10, 11, 12, 13 instead of 1, 3, 6, 10, 11. and 21, 23, 26, 27.

Thanks.