These are a few titles from our ES and MHS collections. The ES library books have an E before the call number, and will be for advanced elementary school readers, so these are good for MS students.
Find more by entering "slavery" in the main search field of the library catalog.
You could also browse the 300s section (this will have slavery in different time periods and regions) and the US history section, 973s.
These are a few picture books with characters who were slaves. To find more, enter "civil rights" in the main search field of the library catalog, and navigate to the end of the list. Look for books with the call number prefix PIC. You can also enter "African American fiction" and read through the summaries of the books with the call number prefix PIC.