The group is unquestionably in trouble now and their detection is imminent. How does the author maintain suspense, considering that the reader knows that the group's cover will be blown?

I don't think that the author does anything overly special to keep the suspense high at this point. Readers have been reading 11 previous chapters that are about David and his group of friends hiding their talent from Waknuk society. Chapter 12 is the chapter to look at for this question. It begins with David preparing to leave, but he is interrupted and forced to go before being fully ready. That definitely increases tension because readers realize that the premature departure is likely to cause more trouble than good. Additionally, we learn early in the chapter that Katherine and Sally have been captured. That's bad to begin with because we know that David might be next; however, their capture is made worse because they shut their minds to the rest of the group. Now there is a huge sense of the unknown going on, and reader imagination begins to build. Those two characters are eventually tortured, and they give up the names of other telepaths. Another suspense-building moment in this chapter is the fact that Rosalind is forced to kill someone. Finally, the chapter ends with a great deal of suspense because David and the others running away with him have been classified as non-humans. This means that anybody can shoot them on sight.