Sending, receiving, and organizing email in Outlook.com
Are you using a school assigned email address? Or an organization assigned email address?
That error normally means Microsoft 365/org tenant has blocked the your account from sending mail because Exchange Online detected suspected spam activity or the account exceeded an outbound sending limit. It is usually not a Gmail problem.
Microsoft doesn't manage organization users directly. Each organization manages their users. You need to contact the organization IT support/admin user. They need to check https://security.microsoft.com/restrictedentities to review why it is blocked and whether it can be unblocked. That's their decision to make, not Microsoft's.