The episode saw teacher Jenna Garvey (Sheridan Smith) discover who had framed her for having an affair with a pupil.
It was revealed that the culprit was in fact Brian, the partner of her friend Pauline – who was a former colleague known as Arnold Cleary.
Cleary had served time in prison after Jenna reported him for having sex with a student, losing his family in the process.
He therefore set out to frame Jenna as an act of revenge.
In a final, shocking twist, Arnold is seen walking into the path of oncoming traffic during a confrontation with Jenna, and is hit by a car.
Fans are not shown whether or not Jenna was able to clear her name, or what happened in the aftermath of Arnold’s death.
Viewers shared their thoughts about the ending on social media, with some expressing shock at the twist and others frustration at the lack of closure for the main storyline.
“Terrible ending to The Teacher!” wrote one person.
“Well I just wasted 4 hours of my life watching The Teacher,” wrote another, who described it as the “worst ending since Game of Thrones”.
“Have I seriously just wasted 4 nights of my life for that awful ending of The Teacher?” asked someone else.
“I feel so disappointed. I’ve loved The Teacher all week.... all week!!!! It’s been gripping, it’s been bloody brilliant.... but that ending.......bloody hell. What rubbish,” complained another viewer.
The entire series can be streamed now on My5.