The 38-year-old plays Jack Rimmer, head of a fictional failing secondary school, in the soap, due to be screened for the first time next week.
However, he said school life was not as bleak as it is painted - especially after reading the pages of The TES.
Mr Merrells said: "You might think from the media that everyone is doing crack cocaine, being stabbed, and that there's no discipline in schools.
But I've heard a lot of positive stories and what strikes me is that the kids can be very warm."
Life at Waterloo Road, a hard-hitting drama written by Ann McManus, an ex-teacher, and based in a sink school in Rochdale, Lancashire, was never going to be simple.
Attention-grabbing storylines include a love triangle involving a male teacher and two female colleagues, and an attack on a deputy head by an angry father.
"It's good to have the opportunity to play a frazzled, really quite troubled, semi-alcoholic man," Mr Merrells said. "His instincts are good, but he has been battered down by the bureaucracy and the way the school has gone downhill."
He drew inspiration from his teacher friend and prepared for his role by reading articles in The TES. He also watched the Channel 4 TV series The Unteachables, which showed a group of teenagers with behavioural problems being taught by a team of experts.
But he hated his own secondary school - a boys-only comprehensive, now closed, near Woodford, Essex. "It was extremely rough and horrible," he said. "Because it was just for boys it was all about hierarchy and fighting."
Holby City star Angela Griffin, who plays Kim Campbell, head of pastoral care at Waterloo Road, said she loved her primary and middle schools so much that she could be very cruel to any teacher who disappointed her.
"Any supply teacher who did not walk in with confidence got taken apart,"
She said it had never occurred to her before her that teachers did not always get on with each other.
"It's like any other workplace."
Waterloo Road is on BBC1 from Thursday March 9 at 8pm