I need to write two 15 line poems about the novel Crabbe by William Bell. One poem must be about a major character and one about a minor character....

This sounds like an interesting assignment. The first step should be to choose the two characters. Choosing Crabbe as the main character would be practical and give you the most to write about. He also has a very clear journey from the time he runs away to the time he comes home, so tracing it would be fairly simple as long as you don't try to include everything that happened to him while he was gone. As for the minor character you choose, you might consider one of his parents or someone else affected by him running away. Just as he goes through multiple feelings and experiences while he is gone, so do those who are left at home and worried about him.

Since each poem must be fifteen lines long, consider breaking each poem up into three sections. For example, the first five lines examines the beginning of Crabbe's journey, the next five look at the middle of his journey, and the last five examine the resolution of his journey. 

Avoiding acrostic poetry should be fairly simple, as this would probably take you more work to actually craft. If you want the piece to rhyme, consider a format and rhyme scheme. Here is a link to a website that gives some pointers on why you might choose to rhyme and how to begin to do so: Creative Writing Now.