Bang, bang, you're dead by Narinder Dhami

Corgi, 2009.
(Age: 12+) Highly recommended. Mia and her twin brother Jamie have much to contend with. Their mother suffers from bipolar disorder and refuses to seek help. When the twins' beloved grandfather dies their Mother's condition spirals out of control. Mia copes as best she can, but Jamie, filled with anger and resentment, seems to be going off the rails. Then one morning Mia's school is subjected to a siege and she is convinced that the crazed gunman holding class 9D hostage is her own brother. Mia discovers depths she never knew she had in an effort to reach Jamie and end the siege peacefully.
This is a tense, smouldering novel that switches between past and present to gradually reveal the events that have set Jamie on this disastrous path to self destruction. However all is not as it seems and from the start Mia continually doubts whether the gunman is actually Jamie. Would her beloved twin brother really be capable of such evil?
Just as we are nearing the explosive finish Dhami lifts the whole plot to a higher plane with a superb twist that makes the reader re-evaluate everything they have just read.
With believable characters and fast moving cinematic action, this is first class writing that will grip you round the jugular and leave you both exhausted and exhilarated. I always find it a challenge to suggest an age range for a novel, as twelve and thirteen year olds vary so much (just as forty year olds do I suppose! However, with the bleak subject matter of major depressive illness and a school siege you may want to recommend this to older readers.
Claire Larson