I'm guessing your Gmail account was provided for you by your school. If so, the system administrator might have disabled the ability for users to change their profile pictures. 

But whoever is managing GSuite (the administrator) at your school should be able to help you.
First, find out who the administrator is. It might be ([source][1]):
- The person who gave you your username
- Someone in your IT department or Help desk (at a company or school)
- The person who manages your email service or web site (in a small business or club)

The tell the administrator that he or she needs to log in to GSuite's administration console and [change or reset your profile picture from there][2].


  [1]: https://support.google.com/a/answer/6208960?hl=en
  [2]: https://support.google.com/mail/thread/3405984?hl=en