One person complained that the sponsored result on the search engine misleadingly implied that the charity, which provides an employment-based training programme for trainee teachers, supplied qualified teachers to schools.
The advert said: "Searching for a teacher? TeachFirst provides outstanding teachers for a two-year commitment."
In a ruling published today, the watchdog upheld the complaint and banned the advert from appearing in its current form. The ASA told TeachFirst in the ruling to ensure its adverts did not state or imply that participants in the programme were already qualified teachers.
"Further information should have been included in the ad to qualify clearly that it was a teacher in training who was being offered," the ASA ruling said. "Because that information was omitted, we concluded that the ad was misleading."
The regulator said that although search engine adverts were significantly limited by space, this did not negate the charity’s responsibility to ensure it did not mislead potential schools about the type of ‘teacher’ being offered.
Schools seeking teachers were likely to be aware that TeachFirst was a charity that ran a programme for training teachers in partnership with schools, the ASA said, but they were unlikely to be aware of the process involved and whether the teachers referred to in the advert had already been through the training programme.
In a statement responding to the ruling, James Westhead, executive director for external relations at TeachFirst, said the advert had not been used for several months.
"It is disappointing that that ASA believes we got the content of the advert wrong on this occasion," he said. "There is a challenge around the nature of a Google advert, which is very limited in size.
"There was never any intention to mislead and nor do we believe anyone was. We have taken on board the views of the ASA and will be mindful of this in developing future advertising."
Westhead said the advert was targeted at school leaders and was designed to attract traffic to the charity’s website, which has a "wealth of information" about TeachFirst’s approach, the role of the school in the training and the work teachers do in the classroom.
"No school leader can enter into a partnership with us without fully understanding the two-year programme and what it means for their school," he said.