The bottom of the cake is covered in brown fondant that we marbled with dark browns to give it a woodgrain look. This is the desk on which the book, paper, pencil, and calculator sit.
The Calculus book is two layers of cake covered in blue fondant. We used a letter stencil to paint on the letters with buttercream, and used the same color buttercream to paint the pages of the book with thick brushstrokes.
The sheet of paper is white fondant rolled paper thin. We then watered down a little bit of blue and red food coloring, dipped a long string of dental floss in it, and used it to make the lines on the paper. We used a tiny circle fondant cutter to cut the holes in the paper.
The writing on the paper was piped on in blue royal icing. Make up your own integral or derivative or whatever other geeky math problem you like. Since he was turning 18, the solution to the integral on the paper is 18 (I thought it was cute...)
The pencil is just rolled, colored fondant. The calculator is a big chunk of fondant shaped into a TI-84, painted black and grey, with fondant buttons.
The book, calculator, and pencil are attached to the desk with buttercream, but you could also use royal icing.
This basic design could easily be tailored to any other subject, with any other accessories!