In Office 365 for student e-mail you cannot change the sender's address due to settings made for security reasons.

If you have another e-mail account, you can send the e-mails there by using the "Connecting to an e-mail account button".

　　By using Microsoft: Outlook on the web, you can make this connection for Office 365. In some cases it cannot be used due to the settings of the e-mail address provider.

Also be careful because it may cause problems related to e-mail sending.