A former violin teacher who measured his teenage students' bare breasts while fitting them for shoulder rests has been convicted on more than two dozen sexual and indecent assault charges after Ontario's top court found it was not necessary to prove he acted with a sexual purpose.

The Court of Appeal for Ontario overturned Claude Trachy's acquittals on those charges but upheld them on a number of sexual interference and sexual exploitation charges, which do require sexual intentions for a conviction.

In a unanimous decision released Tuesday, the appeal court says the trial judge made an error in law by considering sexual intentions as a criteria for all the charges.

Prosecutors had appealed the acquittals largely on this issue, arguing the trial judge oversimplified the allegations to focus exclusively on whether the Chatham, Ont., man acted with a sexual purpose.

The charges relate to incidents that took place in the 1970s through the early 1990s.

Court documents filed in the appeal said the complainants testified they were asked to remove their shirt and bra on the left side so that Trachy could measure them from collarbone to nipple.