I previously used PS 2015, and it worked fine with no bugs or anything. Well I just upgraded to 2018 and it says im on the trial, I never had to sign in with and account before, because the school bought a license for Photoshop. Even my brothers is working like it should, but i turned off my wifi and it instantly the free trial thing popped up saying 7 days left. Please help me 
i am confused because we both did the same thing.
Then you need to contact your schools support service. It may well be, that they need to renew your license. It seams to me, that your school has either taken away your license or that your license expired.
As we have Creative Clouds for teams, I for my part can take away licenses from members of my team and allocate that license to someone else. I can do that, because I'm the attributed team administrator. I can do that without touching the user's PC. My team members need to sign in with their Adobe id that they have created. This is workstation independant. But I suppose there are also licenses attributed to workstations.
Here are the different possibilities your school has: Source: Deploy & Manage | Adobe Enterprise )
For Adobe-hosted, user-managed accounts that are created, owned, and controlled by individual users.
For Adobe-hosted, enterprise-managed accounts that are created and controlled by IT admins. Your organization owns and manages the accounts and all associated assets.
For enterprise-managed accounts with identity profiles provided by your organization's Single Sign-On (SSO) identity management system. IDs are created, owned, and controlled by IT. Your organization owns and manages all accounts and associated assets.
One of those options have been used by your school.
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