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.
So what is the concern? There are three big concerns, actually. Our teacher "caution light" should start blinking if we are using lower_level_thinking worksheets (Example: "Check the box next to the correct answer."), if we are using too many worksheets or if we are using worksheets as classroom busy work.