Be careful when using groups because if you forget that your spreadsheets are grouped and then proceed to add or change figures on a worksheet, all of the worksheets in the group will have the same figures. To remove a group, simply click on a sheet that is not in a group. If all of the worksheets are grouped, right_click on any tab and choose Ungroup Sheets. Keep an eye on the title bar prior to making changes and Excel will tell you if the sheets are grouped.
As far as using worksheets for busy work, the verdict is in. It is destructive to classroom learning to assign worksheets to simply keep students occupied. Busy work creates monotony, causes boredom and increases the likelihood of behavior problems. To know if you are on the right track with worksheets in your classroom, answer (honestly) these simple questions: Do my students groan when I hand out a worksheet? (The answer should be no.) Are my lesson plans based on worksheets? (The answer should be no.) Do I feel anxiety if I don't have worksheets copied? (The answer should be no.) Are students excited about learning in my classroom? (The answer should be yes!).