Inspectors say too many pupils suffer from bullying during their school lives.
A report published today by Estyn also says schools have a responsibility to tackle bullying in all forms under the Education Act 2002, but the ways in which schools deal with bullying varies widely.
'Action on bullying', found that even schools with good strategies to address the problem don't have a common understanding of how important it is to focus on groups of pupils with a higher-than-average risk of being bullied.
They include gay, lesbian and transgender pupils, those with a disability and pupils from a minority ethnic background.
Wales' Education Minister Huw Lewis says the good standards in English highlighted in Estyn's report are 'encouraging' - but conceded improvements need to be made.
We recognise that improvements need to be made, particularly in relation to standards of writing and the performance of pupils from disadvantaged backgrounds. Estyn's comments about poor quality marking and feedback also need to be addressed. This is not good enough and has to change.
The challenge now is to raise standards across the board by taking on the recommendations in this report and by learning from those schools where best practice is being observed. Our Literacy Programme will help ensure that the encouraging improvements that have been observed continue in the years ahead.
The Chief Inspector of Estyn says the rate of progress in reading and writing is 'still too slow' for pupils aged seven to 14 in Wales.
Reading and writing are the key to success in all areas of the curriculum. Despite the improving trend in the standards of English, the rate of progress is still too slow for 7-14 year-olds in Wales to catch-up with other areas of the UK. Inaccuracies in spelling, punctuation and grammar reduce the quality of writing and affect standards.
However, there are schools that have been successful in raising standards in English and I urge others to download the report and follow the lead outlined in the best practice case studies.
Pupils in Wales aged seven to 14 are continuing to show a weakness when it comes to spelling, grammar and punctuation, according to the education inspectorate Estyn.
In a report published today, Estyn said concerns remain about standards in writing and higher reading and that pupils from disadvantaged backgrounds do not achieve as well as their peers.
But the report also found the majority of pupils aged seven to 14 achieve good standards in English overall, with praise for pupils in English lessons speaking clearly during discussions and responding well to a wide variety of texts.
Brecon High School has been placed in special measures, after the education watchdog judged it had not made enough progress on "important weaknesses" at the school.
Estyn visited the school in February and found continued problems with GCSE results, attendance, marking and management at the school.
Inspectors will now monitor Brecon High every term.
Chair of Governors, Councillor David Meredith, said the school "has already drawn up an action plan to ensure that Estyn’s concerns are addressed as a matter of urgency. The school has had a difficult history in recent years, and I am confident that this is now firmly behind us."
Another Powys secondary school - John Beddoes in Presteigne - was placed in special measures in December 2012, and will soon be closed.
Estyn says that, around Wales, there are currently eight primary schools, eight secondary schools and five pupil referral units in special measures.
The education watchdog Estyn says nearly a quarter of the secondary schools inspected last year were judged as 'unsatisfactory' - the lowest grade.
Its report, published today, warns school standards are not improving as they should be.
Responding to the Estyn Chief Inspector's Annual Report, the largest teachers' union in Wales said:
It is regrettable that, once again, the Chief Inspector has used the Annual Report to pander to Welsh Government policies and initiatives and apportion blame for any lack of impact on the profession.
This report will do nothing to restore Estyn's credibility with teachers.
Teaching union ATL Cymru has called on the Welsh Government to improve professional development for teachers and school leaders.
Responding to the annual report from education watchdog Estyn, Dr Philip Dixon, Director or ATL Cymru, says "too much seems at a standstill and some sectors are going backwards. However there are definite signs of hope."
It would easy to become despondent reading the Chief Inspector's Report of progress made in the last year.
Too much seems at a standstill and some sectors are going backwards. However, as one reads deeper, there are definite signs of hope.
There is some excellent, innovative practice being developed and most schools which have been revisited by Estyn are showing improvement.
It's also heartening to read that some schools are reducing the effects of poverty quite considerably.
The report highlights the absolute crucial importance of good quality professional development for staff and leaders.
The Welsh Government should take heed and prioritise that need.
The Chief Inspector's observation about the lack of attention given to pupil's views on what and how they learn is a timely reminder that children and young people are at the heart of the system, and that must never be forgotten by anyone involved in education.
The Welsh Government has insisted "we are making progress" on improving school standards.
The annual report from education watchdog Estyn, published today, warns that "standards of education in Wales have not improved in the main."
The Welsh Government admits "building an excellent education system will take time."
We welcome the Chief Inspector’s comprehensive report and thank Estyn for their work.
We’ve been honest and up-front about the challenges facing our education system and it’s clear from this report that we must continue to work together to improve key areas such as teaching, assessment and literacy and numeracy.
Building an excellent education system, which is the ambition of everyone in the sector, will take time but we are not complacent and we are making progress.
We know there is a lot of good practice across Wales and, as last year’s GCSE results showed, we are closing the gap with England when it comes to performance.
We are determined to put the right policies and initiatives in place to create an education system that truly delivers for our young people.
We will now consider the report and will respond formally in the plenary in March.
Estyn's Chief Inspector has said the watchdog is "very concerned" about the standards in Wales' secondary schools.
Ann Keane admitted: "I had hoped to see better signs of improvement across the board."
23 per cent of the secondary schools inspected in the last year were judged as 'unsatisfactory', up from 14 per cent the previous year.
She told our reporter Tom Sheldrick that the common problems in those schools were weaknesses in leadership, policies on marking and absenteeism not being applied consistently, and not enough being done to help particular groups, such as children from deprived backgrounds.