South Yorkshire Police sent teenagers into 12 venues across the city centre looking for those willing to serve alcohol to customers under 18 years old.
Four failed the test.
A number of licences premises in the city centre have signed up to the 'Challenge 25' policy, where staff should ask for proof of age for anyone attempting to buy alcohol if they feel they look under 25 years old, although customers can legally buy alcohol at 18.
A South Yorkshire Police spokesman said: "Officers conducted a test purchase operation within the city centre. "Police volunteers under the age of 18 attend various licensed premises accompanied by police officers and attempt to purchase alcohol. "It is an offence to sell and to allow the sale of alcohol to persons under the age of 18 years old. "Premises that pass are sent letters praising them of their 'Challenge 25' policy and those that fail will be action planned and re-visited within three months. "Unfortunately if one of our volunteers are served then the person serving is issued with a fixed penalty ticket for Â£90, although this can be waived if the person attends an under age sale prevention course within 10 days. "We tested 12 venues within the city centre and four of these failed. "We are committed to making the city centre safe and working with licensed premises to challenge people who look under the age of 25 and encourage those who work in these venues to ask for identification to prove their age."