Pipe major Douglas McArthur, 47, who is also a fire chief, was also placed on the sex offenders register after the attack in August 2013.
His victim, who was 16 at the time, described McArthur's sentence as "inadequate". She added: "McArthur should have been jailed for what he did to me."
McArthur carried out the assault after giving the teenager a lift to the ferry terminal at Millport on the Isle of Bute, Kilmarnock Sheriff Court heard.
He resigned from the band and is under investigation by the Scottish Fire and Rescue Service, where he is a Watch Commander in charge of Millport's volunteer brigade.