Compiled at the Penn Libraries, Historical Newspapers Online is a table listing free online newspaper archives. And, you are going to want to share this list with your history teachers and your serious young historians.
Arranged by state, the list is curated by Research and Instructional Services Librarian Nick Okrent, who shares his strategy aggregating this useful research content:
This table provides a list of historical U.S. newspapers that are available online at no cost. Newspapers available for free through Google News Historical Archives and Newspaperarchives.com are listed individually as I identify them. Newspapers available through Chronicling America and state digitization projects are usually listed as a group. For instance, under “Wyoming” I have not listed every newspaper digitized in the project but simply described what is available.
For me this was a particularly opportune discovery. Right before the junior Civil War unit, I was excited to discover Pennsylvania Civil War Newspapers (including Philadelphia papers) 1847-1874 and The Philadelphia Inquirer Civil War Archive.
Nick also notes that Google News Archives has finally created a list of all the newspapers it has ingested, including a variety of newspapers not included ion the U. Penn list. Also remember Google News Timeline for free access to historical news content.