To add teachers from a spreadsheet, you simply need to copy the column of your spreadsheet containing their email addresses. Do not copy the column header. Then in the "Teachers" section, click "Import" and paste your email addresses into the form. Follow the instructions to complete the import. Be sure to tell your teachers to look for an email invitation.
TIP: Be sure to have them check their junk mail and spam folder if they don't the emails in their inboxes.