Merging Two Excel Spreadsheets

I have the same data from two different schools that I need to merge into one spreadsheet. I have previously been able to cut and paste. Now I get the following message:

To paste all cells from an excel worksheet into the current worksheet you must paste into the first cell A1 or R1C1.

The problem with this is that it replaces the information from the first spreadsheet instead of adding to it. I have been cutting and pasting all year with no problems. However, I stared getting this message this week.