Hearts, Dead And Alive by Kate McLachlan

When fifth grade teacher Kimberly Wayland finds a human heart in the middle school dumpster, she has some explaining to do. Like why she was in the dumpster in the first place, and why she didn't tell the police about her gruesome find. But after giving the police a fake alibi, explaining is the last thing Kim wants to do. Instead, with the help of her friends - hot 'best friend' Becca, co-worker 'lesbian wanna-be' Annie, and lawyer 'stickler-for-rules' Lucy - Kim sets out to solve the mystery of the missing heart. Along the way, she unexpectedly solves another mystery, the mystery of her own heart.