Appearing at Southampton Crown Court today, he admitted 10 sexual offences against one of the boys including two counts of raping the youngster.
The offences occurred between September 2013 and November last year, when the victim was aged between seven and eight.
Dennis' second victim was indecently assaulted by the former teacher in the late 1990s, the court heard.
The former teacher hung his head in shame as he was told he will be sentenced in November for 20 charges in total, including possessing indecent images of children and extreme animal porn.
He also admitted a sexual assault charge by touching the child indecently and assault by penetration.
In addition, Dennis, of Eastleigh, Hampshire, admitted two counts of sexual activity with the boy.
In relation to the second victim in the late 1990s, Dennis admitted one count of indecent assault and a further count of indecency with a child.
Originally he denied these two charges and was due to stand trial over them, but changed his plea today.
Dennis also admitted four counts of making indecent photographs of a child and three counts of possessing indecent images.
He also admitted a further charge of possessing an extreme pornographic image.
Judge Nicholas Rowland told him: 'Your case will come back on November 5 for sentencing and a pre-sentence report is being ordered to consider the question of dangerousness.'
Kerry Maylin, prosecuting, said Dennis' level of offending was 'clearly very serious', adding: 'It's all the more serious considering the age disparity.'
In relation to the more recent victim, Dennis admitted two counts of rape and four counts of causing or inciting the boy to engage in sexual activity.