Yellow is for Sunflowers by Kathryn Freeman

Dean Baxter was everything a sensible thirty-two-year-old teacher should avoid. Five years younger, cocky, with a playboy reputation. Oh and guardian to Tom, one of her most promising students. But Lia was a rebel at heart.
Lia Stapleton was everything that was out of reach for an uneducated garage mechanic who’d made too many mistakes. Cambridge-educated, gorgeous, from a well-heeled background. And his brother’s teacher. But Dean couldn’t resist her.
As Dean and Lia’s tentative relationship begins, can it survive Tom’s disapproval, her parents’ interference. And his past mistakes.