I would recommend iron on interfacing. Because the bag is going to be used for heavy duty school books, I'd also recommend that you get some fleece or quilted calico and sew this in between the outer (with the interfacing attached) and your lining. This will provide some protection from bumps for your books. I know how expensive those things can get!
Be careful when purchasing cheap interfacing - often is is very brittle, almost like paper and it makes your bag feel crunchy. A great tip when applying fusible interfacing is to turn the temperature down - often when you apply interfacing it bubbles - this is because the heat is too high.