Robert Rixon was found guilty at Cardiff Crown Court of 28 charges of sexual assault, including one of rape.
Rixon, from Porthcawl, in Bridgend county, abused three girls aged 11-13 in Porthcawl Comprehensive School before offending again at another school years later.
The first victim came forward in 2018, and four others identified themselves.
After the first offences between 2000 and 2005, Rixon hired a pool at Heronsbridge School in Bridgend from 2013 to 2015 to run a lifesaving club.
Here he sexually assaulted two girls of the same age.
An investigation was launched three years later after one of the women came forward and reported that she had been abused by him as a girl.
This led to police identifying four other victims which resulted in an in-depth and complex inquiry, revealing a number of allegations of serious sexual abuse.
Det Con Tony Daniel of South Wales Police paid tribute to the “tremendously brave” victims.p
“To see Rixon behind bars will hopefully enable these five women to move on with their lives in the knowledge that he has finally been brought to justice for his heinous and indefensible crimes,” he said.
“Help and support is available for all victims of sexual violence and we encourage any victims, no matter how long ago the offence happened, to come forward.”
