Change the Response Destination
In Google Forms, select Responses and choose Change response destination...
You will want to create a new spreadsheet and maybe add the school year to the title. Select Create.
But I already sent out the form...now what?
If you already sent out the form, you may have your new responses, but they are below all of the rows you already deleted. It may look something like this:
Not to worry, you need to remove the rows entirely! Select the row heading (the numbers on the left). If it is a large amount of rows, select the first row you want to delete scroll down to the last row you want to delete, hold down the Shift Key & Select that row. The entire range of rows should be selected. Right Click and a similar menu should appear. Select Delete Row.
Now, your sheet should be fixed and you can Keep Calm, Click Undo!