The 33-year-old, who is the younger brother of Harlequins number eight Nick, joined Sale in 2011 after six years at Northampton.
Easter won the European Challenge Cup and reached the Champions Cup final at Saints after joining from Nottingham.
"I've enjoyed my time at Sale, I didn't really want to go anywhere else," said the back-row forward.
"It was the right time to leave on my own terms, which not many people do.
"I've had the chance for these last few games just to enjoy and savour them and I've been really lucky."
He will now take up his teaching position at Wrekin College in Shropshire after the final game of the season at Newcastle on Saturday.
"The last few years, I've been tailoring my career towards teaching, so taking a few courses, getting into schools, teaching and coaching," he added.
"The opportunity just came up at the right time. It felt like the right time for me and my family."

Pritchard, a former England Under-21 international, has agreed a four-year deal at Carrow Road.
The 23-year-old has scored a total of 20 goals in 100 matches on loan away from White Hart Lane at Peterborough, Swindon, Brentford and West Brom.
Jones, 30, has signed a two-year deal after leaving Portsmouth.
He has made more than 400 career appearances for clubs including Exeter, Peterborough and Crawley and will fight for a first-team place alongside John Ruddy and Michael McGovern.
Pritchard told the club website: "I feel I need to go out and prove myself again in football. I'm 23 now so it's about time I had my own path really."
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Sale Sharks forward Mark Easter has announced he will retire at the end of the season to take up a teaching role.
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Norwich City have signed midfielder Alex Pritchard from Tottenham Hotspur for an undisclosed fee and goalkeeper Paul Jones on a free transfer.