Colin Bell, 45, of Raven Court, Ruislip, London, taught at Westcliff High School for Girls until his arrest.
He also faced four charges related to indecent images of children, one of which he admitted.
The Crown Prosecution Service decided not to proceed with the remaining three counts. He was given an absolute discharge at Basildon Crown Court.
Mr Bell was arrested in August 2015 and in June the following year was charged with four counts related to indecent images of children and three of sexual touching of a girl under 18.
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He was acquitted of all the sexual assault charges by a jury on 31 March.
Immediately after the trial he admitted one count of making an indecent photograph or pseudo-photograph of a child, after four indecent images were found on a DVD, a Crown Prosecution Service (CPS) spokeswoman said.
"We considered this plea and took into account remarks by the judge regarding the remaining charges and the likely sentence they would attract.
"We decided to accept the guilty plea and offer no evidence on the other charges.
"The judge sentenced Mr Bell to an absolute discharge," she added.
Although an absolute discharge means no further action will be taken against Mr Bell, he will have a criminal record.
A former teacher at an all girls school in Essex has been acquitted of sexually touching a girl under 18 years of age.