You haven't given us too much to go on, but this sounds like The Demon Headmaster Strikes Again (published 1996), part of the Demon Headmaster series.
Dinah's father is headhunted for a new job at the Biogenetic Research Centre, but little do they know that the Demon Headmaster is the director. This time his lust for power sees him meddling with evolution itself. He has created an Evolution Accelerator: a machine to speed up the evolution process and produce the perfect human, beginning with the development of a fast-growing poisonous creeper that attacks Lloyd. But for this, he needs Dinah's DNA - combining it with a lizard to create Eve, a human/lizard hybrid who possesses Dinah's intellect but lacks her emotions - and then put her into terrible danger, pitting her against Eve to test his theories. This became series two of the television adaptation.
The protagonists are a group of teenagers (Dinah, her adoptive brothers Lloyd and Harvey, and their friends Ian, Mandy, and Ingrid) who form a secret society known as SPLAT. In the original book of the series, they were pitted against the headmaster of their school, who possesses the power of hypnotism; in each subsequent book, he reappears up to mischief again and trying to take over the world. They did have black and white illustrations, at least in the editions I read. I'll try to track down some of the images for you.