TEACHING and achievement are "inadequate" at Beaufort Community School, inspectors found.
The secondary school in Tuffley was judged as having "serious weaknesses" in an Ofsted inspection last month.
The education watchdog said achievement is inadequate in English and many pupils' writing skills lack the necessary detail to get high exam grades.
Inspectors also found students are not making enough progress due to "inadequate" teaching, and teachers "often plan work that is either too hard or too easy".
Behaviour needs to improve, and marking "does not always tell students how to improve their work".
Ofsted felt positive changes have already been made at the school, including raising attainment in maths and science and improvements in attendance.
They also noted good and improving teaching, found pupils to be "polite and courteous" and found stronger progress and higher standards in the sixth form.
The school has a new headteacher in place, David Bishop, who joined the school in September 2012. The Ofsted inspector is confident that the new head "knows exactly what else needs to be done and is starting to make sure it happens".
"I am extremely pleased that the inspection team noted the positive changes already made by the school and the impact on standards of achievement and pupils' attitude to learning this has had," said Mr Bishop.
"We are very proud of all our pupils and will continue to put them at the centre of everything that we do.
"The report also gives us clear guidance on what needs to be done further, to ensure the continued improvement of Beaufort Community School. Plans are already in place to ensure that all the action points we have been given are met and that progress towards these is rapid. With the continued support of Gloucestershire County Council and the overning body, I have no doubt that we will deliver the improvements required."