Hi Madelyn,
This happens when you are actually enrolled in a course that remains active at that school. So if a teacher from a school added you to their course, or if a PD course was created in a different school's subaccount, you will receive Global Announcements from that school.
You may want to look through all the courses in which you are enrolled and try and identify which courses are not from your school, and ask the "owner" of that course to remove you. If you can't identify which are the "culprit" courses -- I recommend you reach out to your Canvas System Admin at your district.
In my district we try and remind people, also, to Conclude/End the manually created courses at the end of the school year (and duplicate them for the next school year). In this way you don't worry about people remaining in active courses after they leave a school.